AJAX error on "add another item" unlimited field values

We have several content types on the site that utilize fields that allows for multiple values per field. In the last two days we've noticed we cannot add another item. The log yields the following error:

Notice: Undefined index: #ajax in ajax_form_callback() (line 395 of /home/~/public_html/includes/ajax.inc).

No new field pops up after clicking the button, even though the loading throbber does show up. If you try to save the node after trying to add another item, it tells you "The content on this page has either been modified by another user, or you have already submitted modifications using this form. As a result, your changes cannot be saved."

Obviously, there is nobody else trying to edit the node, so not sure where that issue is coming from. The return from the ajax request returns:

[{"command":"settings","settings":{"basePath":"\/","pathPrefix":"","ajaxPageState":{"theme":"lgride","theme_token":"Uff0T2jG6L0jikC5i5U5On7S1kVB2DRN5nxl_sTLiDE"}},"merge":true}]

Instead of something like this (example from a different one of our working Drupal sites):

[{"command":"settings","settings":{"basePath":"\/","pathPrefix":"","ajaxPageState":{"theme":"mix_and_match","theme_token":"3DMaLhruo79-eXi4IFKVqDRmY1hAQ_ZtZEW2QUqIdJY","jquery_version":"1.10"},"shadowbox":{"animate":1,"animateFade":1,"animSequence":"wh","auto_enable_all_images":1,"auto_gallery":1,"autoplayMovies":true,"continuous":0,"counterLimit":"10","counterType":"default","displayCounter":1,"displayNav":1,"enableKeys":1,"fadeDuration":"0.35","handleOversize":"resize","handleUnsupported":"link","initialHeight":160,"initialWidth":320,"language":"en","modal":false,"overlayColor":"#000","overlayOpacity":"0.85","resizeDuration":"0.35","showMovieControls":1,"slideshowDelay":"0","viewportPadding":20,"useSizzle":0},"jquery":{"ui":{"datepicker":{"isRTL":false,"firstDay":"0"}}},"datePopup":{"edit-pubdate--2-datepicker-popup-0":{"func":"datepicker","settings":{"changeMonth":true,"changeYear":true,"autoPopUp":"focus","closeAtTop":false,"speed":"immediate","firstDay":0,"dateFormat":"yy-mm-dd","yearRange":"-6:+1","fromTo":false,"defaultDate":"0y"}},"edit-pubdate--2-timeEntry-popup-1":{"func":"timeEntry","settings":{"show24Hours":true,"showSeconds":true,"timeSteps":[1,1,1],"spinnerImage":"","fromTo":false}},"edit-pubdate--2-datepicker-popup-2":{"func":"datepicker","settings":{"changeMonth":true,"changeYear":true,"autoPopUp":"focus","closeAtTop":false,"speed":"immediate","firstDay":0,"dateFormat":"yy-mm-dd","yearRange":"-6:+1","fromTo":false,"defaultDate":"0y"}},"edit-pubdate--2-timeEntry-popup-3":{"func":"timeEntry","settings":{"show24Hours":true,"showSeconds":true,"timeSteps":[1,1,1],"spinnerImage":"","fromTo":false}}},"ajax":{"edit-field-report-names-und-add-more--3":{"callback":"field_add_more_js","wrapper":"field-report-names-add-more-wrapper--3","effect":"fade","event":"mousedown","keypress":true,"prevent":"click","url":"\/system\/ajax","submit":{"_triggering_element_name":"field_report_names_add_more","_triggering_element_value":"Add another item"}}},"urlIsAjaxTrusted":{"\/system\/ajax":true},"tableDrag":{"field-report-names-values--3":{"field_report_names-delta-order":[{"target":"field_report_names-delta-order","source":"field_report_names-delta-order","relationship":"sibling","action":"order","hidden":true,"limit":0}]}}},"merge":true},{"command":"insert","method":null,"selector":null,"data":"\u003Cdiv id=\u0022field-report-names-add-more-wrapper--3\u0022\u003E\u003Cdiv class=\u0022form-item\u0022\u003E\u003Ctable id=\u0022field-report-names-values--3\u0022 class=\u0022field-multiple-table sticky-enabled\u0022\u003E\n \u003Cthead\u003E\u003Ctr\u003E\u003Cth colspan=\u00222\u0022 class=\u0022field-label\u0022\u003E\u003Clabel\u003EReporter(s) \u003C\/label\u003E\u003C\/th\u003E\u003Cth\u003EOrder\u003C\/th\u003E \u003C\/tr\u003E\u003C\/thead\u003E\n\u003Ctbody\u003E\n \u003Ctr class=\u0022draggable odd\u0022\u003E\u003Ctd class=\u0022field-multiple-drag\u0022\u003E\u003C\/td\u003E\u003Ctd\u003E\u003Cdiv class=\u0022form-item form-type-textfield form-item-field-report-names-und-0-value\u0022\u003E\n \u003Cinput class=\u0022text-full form-text\u0022 type=\u0022text\u0022 id=\u0022edit-field-report-names-und-0-value--3\u0022 name=\u0022field_report_names[und][0][value]\u0022 value=\u0022\u0022 size=\u002235\u0022 maxlength=\u0022255\u0022 \/\u003E\n\u003C\/div\u003E\n\u003C\/td\u003E\u003Ctd class=\u0022delta-order\u0022\u003E\u003Cdiv class=\u0022form-item form-type-select form-item-field-report-names-und-0--weight\u0022\u003E\n \u003Clabel class=\u0022element-invisible\u0022 for=\u0022edit-field-report-names-und-0-weight--3\u0022\u003EWeight for row 1 \u003C\/label\u003E\n \u003Cselect class=\u0022field_report_names-delta-order form-select\u0022 id=\u0022edit-field-report-names-und-0-weight--3\u0022 name=\u0022field_report_names[und][0][_weight]\u0022\u003E\u003Coption value=\u0022-3\u0022\u003E-3\u003C\/option\u003E\u003Coption value=\u0022-2\u0022\u003E-2\u003C\/option\u003E\u003Coption value=\u0022-1\u0022\u003E-1\u003C\/option\u003E\u003Coption value=\u00220\u0022 selected=\u0022selected\u0022\u003E0\u003C\/option\u003E\u003Coption value=\u00221\u0022\u003E1\u003C\/option\u003E\u003Coption value=\u00222\u0022\u003E2\u003C\/option\u003E\u003Coption value=\u00223\u0022\u003E3\u003C\/option\u003E\u003C\/select\u003E\n\u003C\/div\u003E\n\u003C\/td\u003E \u003C\/tr\u003E\n \u003Ctr class=\u0022draggable even\u0022\u003E\u003Ctd class=\u0022field-multiple-drag\u0022\u003E\u003C\/td\u003E\u003Ctd\u003E\u003Cdiv class=\u0022form-item form-type-textfield form-item-field-report-names-und-1-value\u0022\u003E\n \u003Cinput class=\u0022text-full form-text\u0022 type=\u0022text\u0022 id=\u0022edit-field-report-names-und-1-value--3\u0022 name=\u0022field_report_names[und][1][value]\u0022 value=\u0022\u0022 size=\u002235\u0022 maxlength=\u0022255\u0022 \/\u003E\n\u003C\/div\u003E\n\u003C\/td\u003E\u003Ctd class=\u0022delta-order\u0022\u003E\u003Cdiv class=\u0022form-item form-type-select form-item-field-report-names-und-1--weight\u0022\u003E\n \u003Clabel class=\u0022element-invisible\u0022 for=\u0022edit-field-report-names-und-1-weight--3\u0022\u003EWeight for row 2 \u003C\/label\u003E\n \u003Cselect class=\u0022field_report_names-delta-order form-select\u0022 id=\u0022edit-field-report-names-und-1-weight--3\u0022 name=\u0022field_report_names[und][1][_weight]\u0022\u003E\u003Coption value=\u0022-3\u0022\u003E-3\u003C\/option\u003E\u003Coption value=\u0022-2\u0022\u003E-2\u003C\/option\u003E\u003Coption value=\u0022-1\u0022\u003E-1\u003C\/option\u003E\u003Coption value=\u00220\u0022\u003E0\u003C\/option\u003E\u003Coption value=\u00221\u0022 selected=\u0022selected\u0022\u003E1\u003C\/option\u003E\u003Coption value=\u00222\u0022\u003E2\u003C\/option\u003E\u003Coption value=\u00223\u0022\u003E3\u003C\/option\u003E\u003C\/select\u003E\n\u003C\/div\u003E\n\u003C\/td\u003E \u003C\/tr\u003E\n \u003Ctr class=\u0022draggable odd\u0022\u003E\u003Ctd class=\u0022field-multiple-drag\u0022\u003E\u003C\/td\u003E\u003Ctd\u003E\u003Cdiv class=\u0022form-item form-type-textfield form-item-field-report-names-und-2-value\u0022\u003E\n \u003Cinput class=\u0022text-full form-text\u0022 type=\u0022text\u0022 id=\u0022edit-field-report-names-und-2-value--2\u0022 name=\u0022field_report_names[und][2][value]\u0022 value=\u0022\u0022 size=\u002235\u0022 maxlength=\u0022255\u0022 \/\u003E\n\u003C\/div\u003E\n\u003C\/td\u003E\u003Ctd class=\u0022delta-order\u0022\u003E\u003Cdiv class=\u0022form-item form-type-select form-item-field-report-names-und-2--weight\u0022\u003E\n \u003Clabel class=\u0022element-invisible\u0022 for=\u0022edit-field-report-names-und-2-weight--2\u0022\u003EWeight for row 3 \u003C\/label\u003E\n \u003Cselect class=\u0022field_report_names-delta-order form-select\u0022 id=\u0022edit-field-report-names-und-2-weight--2\u0022 name=\u0022field_report_names[und][2][_weight]\u0022\u003E\u003Coption value=\u0022-3\u0022\u003E-3\u003C\/option\u003E\u003Coption value=\u0022-2\u0022\u003E-2\u003C\/option\u003E\u003Coption value=\u0022-1\u0022\u003E-1\u003C\/option\u003E\u003Coption value=\u00220\u0022\u003E0\u003C\/option\u003E\u003Coption value=\u00221\u0022\u003E1\u003C\/option\u003E\u003Coption value=\u00222\u0022 selected=\u0022selected\u0022\u003E2\u003C\/option\u003E\u003Coption value=\u00223\u0022\u003E3\u003C\/option\u003E\u003C\/select\u003E\n\u003C\/div\u003E\n\u003C\/td\u003E \u003C\/tr\u003E\n \u003Ctr class=\u0022draggable even\u0022\u003E\u003Ctd class=\u0022field-multiple-drag\u0022\u003E\u003C\/td\u003E\u003Ctd\u003E\u003Cdiv class=\u0022ajax-new-content\u0022\u003E\u003Cdiv class=\u0022form-item form-type-textfield form-item-field-report-names-und-3-value\u0022\u003E\n \u003Cinput class=\u0022text-full form-text\u0022 type=\u0022text\u0022 id=\u0022edit-field-report-names-und-3-value\u0022 name=\u0022field_report_names[und][3][value]\u0022 value=\u0022\u0022 size=\u002235\u0022 maxlength=\u0022255\u0022 \/\u003E\n\u003C\/div\u003E\n\u003C\/div\u003E\u003C\/td\u003E\u003Ctd class=\u0022delta-order\u0022\u003E\u003Cdiv class=\u0022form-item form-type-select form-item-field-report-names-und-3--weight\u0022\u003E\n \u003Clabel class=\u0022element-invisible\u0022 for=\u0022edit-field-report-names-und-3-weight\u0022\u003EWeight for row 4 \u003C\/label\u003E\n \u003Cselect class=\u0022field_report_names-delta-order form-select\u0022 id=\u0022edit-field-report-names-und-3-weight\u0022 name=\u0022field_report_names[und][3][_weight]\u0022\u003E\u003Coption value=\u0022-3\u0022\u003E-3\u003C\/option\u003E\u003Coption value=\u0022-2\u0022\u003E-2\u003C\/option\u003E\u003Coption value=\u0022-1\u0022\u003E-1\u003C\/option\u003E\u003Coption value=\u00220\u0022\u003E0\u003C\/option\u003E\u003Coption value=\u00221\u0022\u003E1\u003C\/option\u003E\u003Coption value=\u00222\u0022\u003E2\u003C\/option\u003E\u003Coption value=\u00223\u0022 selected=\u0022selected\u0022\u003E3\u003C\/option\u003E\u003C\/select\u003E\n\u003C\/div\u003E\n\u003C\/td\u003E \u003C\/tr\u003E\n\u003C\/tbody\u003E\n\u003C\/table\u003E\n\u003Cdiv class=\u0022clearfix\u0022\u003E\u003Cinput class=\u0022field-add-more-submit form-submit\u0022 type=\u0022submit\u0022 id=\u0022edit-field-report-names-und-add-more--3\u0022 name=\u0022field_report_names_add_more\u0022 value=\u0022Add another item\u0022 \/\u003E\u003C\/div\u003E\u003C\/div\u003E\u003C\/div\u003E","settings":null},{"command":"insert","method":"prepend","selector":null,"data":"","settings":null}]

Some help would be appreciated. We haven't done any module updates recently to change anything.