Adding custom field results in Ajax error

I was trying to add a block field, but the same thing happens trying to add any kind of custom field.

An AJAX HTTP error occurred.
HTTP Result Code: 200
Debugging information follows.
Path: /admin/structure/ds/ajax/add_field/manage_block
StatusText: parsererror
ResponseText: /[{"command":"settings","settings":{"basePath":"\/","pathPrefix":"","ajaxPageState":{"theme":"seven","theme_token":"mCGDsAt36TQt_duO983PT_iD0aAjVQYEbV0gxfBFYIM","js":{"misc\/machine-name.js":1,"misc\/textarea.js":1}},"ThemeKeyRedirect":{"checkOnce":false,"redirectOnce":false},"machineName":{"#edit-name":{"exists":"ds_field_unique","source":["name"],"target":"#edit-field","label":"Machine name","replace_pattern":"[^a-z0-9_]+","replace":"_","standalone":false,"field_prefix":"","field_suffix":"","suffix":"#edit-name-machine-name-suffix"}},"chosen":{"selector":"select:visible","minimum_single":0,"minimum_multiple":0,"minimum_width":200,"options":{"allow_single_deselect":false,"disable_search":false,"disable_search_threshold":0,"search_contains":true,"placeholder_text_multiple":"Choose some options","placeholder_text_single":"Choose an option","no_results_text":"No results match","inherit_select_classes":true}},"urlIsAjaxTrusted":{"\/admin\/structure\/ds\/ajax\/add_field\/manage_block":true}},"merge":true},{"command":"insert","method":"prepend","selector":"head","data":"\u003Cscript type=\u0022text\/javascript\u0022 src=\u0022http:\/\/local.captconnect.edc.org\/misc\/machine-name.js?okeb9e\u0022\u003E\u003C\/script\u003E\n\u003Cscript type=\u0022text\/javascript\u0022 src=\u0022http:\/\/local.captconnect.edc.org\/misc\/textarea.js?v=7.53\u0022\u003E\u003C\/script\u003E\n","settings":null},{"command":"modal_display","title":"Add a block field","output":"\u003Cform action=\u0022\/admin\/structure\/ds\/ajax\/add_field\/manage_block\u0022 method=\u0022post\u0022 id=\u0022ds-edit-block-field-form\u0022 accept-charset=\u0022UTF-8\u0022\u003E\u003Cdiv\u003E\u003Cdiv class=\u0022form-item form-type-textfield form-item-name\u0022\u003E\n \u003Clabel for=\u0022edit-name\u0022\u003ELabel \u003Cspan class=\u0022form-required\u0022 title=\u0022This field is required.\u0022\u003E*\u003C\/span\u003E\u003C\/label\u003E\n \u003Cinput type=\u0022text\u0022 id=\u0022edit-name\u0022 name=\u0022name\u0022 value=\u0022\u0022 size=\u002230\u0022 maxlength=\u0022128\u0022 class=\u0022form-text required\u0022 \/\u003E \u003Cspan class=\u0022field-suffix\u0022\u003E \u003Csmall id=\u0022edit-name-machine-name-suffix\u0022\u003E\u0026nbsp;\u003C\/small\u003E\u003C\/span\u003E\n\u003Cdiv class=\u0022description\u0022\u003EThe human-readable label of the field.\u003C\/div\u003E\n\u003C\/div\u003E\n\u003Cdiv class=\u0022form-item form-type-machine-name form-item-field\u0022\u003E\n \u003Clabel for=\u0022edit-field\u0022\u003EMachine-readable name \u003Cspan class=\u0022form-required\u0022 title=\u0022This field is required.\u0022\u003E*\u003C\/span\u003E\u003C\/label\u003E\n \u003Cspan class=\u0022field-prefix\u0022\u003E\u003C\/span\u003E \u003Cinput dir=\u0022ltr\u0022 type=\u0022text\u0022 id=\u0022edit-field\u0022 name=\u0022field\u0022 value=\u0022\u0022 size=\u002260\u0022 maxlength=\u002232\u0022 class=\u0022form-text required\u0022 \/\u003E \u003Cspan class=\u0022field-suffix\u0022\u003E\u003C\/span\u003E\n\u003Cdiv class=\u0022description\u0022\u003EThe machine-readable name of this field. This name must contain only lowercase letters and underscores. This name must be unique.\u003C\/div\u003E\n\u003C\/div\u003E\n\u003Cdiv class=\u0022form-item form-type-checkboxes form-item-entities\u0022\u003E\n \u003Clabel for=\u0022edit-entities\u0022\u003EEntities \u003Cspan class=\u0022form-required\u0022 title=\u0022This field is required.\u0022\u003E*\u003C\/span\u003E\u003C\/label\u003E\n \u003Cdiv id=\u0022edit-entities\u0022 class=\u0022form-checkboxes\u0022\u003E\u003Cdiv class=\u0022form-item form-type-checkbox form-item-entities-node\u0022\u003E\n \u003Cinput type=\u0022checkbox\u0022 id=\u0022edit-entities-node\u0022 name=\u0022entities[node]\u0022 value=\u0022node\u0022 class=\u0022form-checkbox\u0022 \/\u003E \u003Clabel class=\u0022option\u0022 for=\u0022edit-entities-node\u0022\u003ENode \u003C\/label\u003E\n\n\u003C\/div\u003E\n\u003Cdiv class=\u0022form-item form-type-checkbox form-item-entities-user\u0022\u003E\n \u003Cinput type=\u0022checkbox\u0022 id=\u0022edit-entities-user\u0022 name=\u0022entities[user]\u0022 value=\u0022user\u0022 class=\u0022form-checkbox\u0022 \/\u003E \u003Clabel class=\u0022option\u0022 for=\u0022edit-entities-user\u0022\u003EUser \u003C\/label\u003E\n\n\u003C\/div\u003E\n\u003Cdiv class=\u0022form-item form-type-checkbox form-item-entities-taxonomy-term\u0022\u003E\n \u003Cinput type=\u0022checkbox\u0022 id=\u0022edit-entities-taxonomy-term\u0022 name=\u0022entities[taxonomy_term]\u0022 value=\u0022taxonomy_term\u0022 class=\u0022form-checkbox\u0022 \/\u003E \u003Clabel class=\u0022option\u0022 for=\u0022edit-entities-taxonomy-term\u0022\u003ETaxonomy term \u003C\/label\u003E\n\n\u003C\/div\u003E\n\u003Cdiv class=\u0022form-item form-type-checkbox form-item-entities-cer\u0022\u003E\n \u003Cinput type=\u0022checkbox\u0022 id=\u0022edit-entities-cer\u0022 name=\u0022entities[cer]\u0022 value=\u0022cer\u0022 class=\u0022form-checkbox\u0022 \/\u003E \u003Clabel class=\u0022option\u0022 for=\u0022edit-entities-cer\u0022\u003ECer \u003C\/label\u003E\n\n\u003C\/div\u003E\n\u003Cdiv class=\u0022form-item form-type-checkbox form-item-entities-entityform\u0022\u003E\n \u003Cinput type=\u0022checkbox\u0022 id=\u0022edit-entities-entityform\u0022 name=\u0022entities[entityform]\u0022 value=\u0022entityform\u0022 class=\u0022form-checkbox\u0022 \/\u003E \u003Clabel class=\u0022option\u0022 for=\u0022edit-entities-entityform\u0022\u003EEntityform \u003C\/label\u003E\n\n\u003C\/div\u003E\n\u003Cdiv class=\u0022form-item form-type-checkbox form-item-entities-flagging\u0022\u003E\n \u003Cinput type=\u0022checkbox\u0022 id=\u0022edit-entities-flagging\u0022 name=\u0022entities[flagging]\u0022 value=\u0022flagging\u0022 class=\u0022form-checkbox\u0022 \/\u003E \u003Clabel class=\u0022option\u0022 for=\u0022edit-entities-flagging\u0022\u003EFlagging \u003C\/label\u003E\n\n\u003C\/div\u003E\n\u003Cdiv class=\u0022form-item form-type-checkbox form-item-entities-og-membership-type\u0022\u003E\n \u003Cinput type=\u0022checkbox\u0022 id=\u0022edit-entities-og-membership-type\u0022 name=\u0022entities[og_membership_type]\u0022 value=\u0022og_membership_type\u0022 class=\u0022form-checkbox\u0022 \/\u003E \u003Clabel class=\u0022option\u0022 for=\u0022edit-entities-og-membership-type\u0022\u003EOg membership type \u003C\/label\u003E\n\n\u003C\/div\u003E\n\u003Cdiv class=\u0022form-item form-type-checkbox form-item-entities-og-membership\u0022\u003E\n \u003Cinput type=\u0022checkbox\u0022 id=\u0022edit-entities-og-membership\u0022 name=\u0022entities[og_membership]\u0022 value=\u0022og_membership\u0022 class=\u0022form-checkbox\u0022 \/\u003E \u003Clabel class=\u0022option\u0022 for=\u0022edit-entities-og-membership\u0022\u003EOg membership \u003C\/label\u003E\n\n\u003C\/div\u003E\n\u003Cdiv class=\u0022form-item form-type-checkbox form-item-entities-WorkflowTransition\u0022\u003E\n \u003Cinput type=\u0022checkbox\u0022 id=\u0022edit-entities-workflowtransition\u0022 name=\u0022entities[WorkflowTransition]\u0022 value=\u0022WorkflowTransition\u0022 class=\u0022form-checkbox\u0022 \/\u003E \u003Clabel class=\u0022option\u0022 for=\u0022edit-entities-workflowtransition\u0022\u003EWorkflowTransition \u003C\/label\u003E\n\n\u003C\/div\u003E\n\u003Cdiv class=\u0022form-item form-type-checkbox form-item-entities-rules-config\u0022\u003E\n \u003Cinput type=\u0022checkbox\u0022 id=\u0022edit-entities-rules-config\u0022 name=\u0022entities[rules_config]\u0022 value=\u0022rules_config\u0022 class=\u0022form-checkbox\u0022 \/\u003E \u003Clabel class=\u0022option\u0022 for=\u0022edit-entities-rules-config\u0022\u003ERules config \u003C\/label\u003E\n\n\u003C\/div\u003E\n\u003C\/div\u003E\n\u003Cdiv class=\u0022description\u0022\u003ESelect the entities for which this field will be made available.\u003C\/div\u003E\n\u003C\/div\u003E\n\u003Cdiv class=\u0022form-item form-type-textarea form-item-ui-limit\u0022\u003E\n \u003Clabel for=\u0022edit-ui-limit\u0022\u003ELimit field \u003C\/label\u003E\n \u003Cdiv class=\u0022form-textarea-wrapper resizable\u0022\u003E\u003Ctextarea id=\u0022edit-ui-limit\u0022 name=\u0022ui_limit\u0022 cols=\u002260\u0022 rows=\u00225\u0022 class=\u0022form-textarea\u0022\u003E\u003C\/textarea\u003E\u003C\/div\u003E\n\u003Cdiv class=\u0022description\u0022\u003ELimit this field on field UI per bundles and\/or view modes. The values are in the form of $bundle|$view_mode, where $view_mode may be either a view mode set to use custom settings, or \u0027default\u0027. You may use * to select all, e.g article|*, *|full or *|*. Enter one value per line.\u003C\/div\u003E\n\u003C\/div\u003E\n\u003Cdiv class=\u0022form-item form-type-select form-item-block\u0022\u003E\n \u003Clabel for=\u0022edit-block\u0022\u003EBlock \u003Cspan class=\u0022form-required\u0022 title=\u0022This field is required.\u0022\u003E*\u003C\/span\u003E\u003C\/label\u003E\n \u003Cselect id=\u0022edit-block\u0022 name=\u0022block\u0022 class=\u0022form-select required\u0022\u003E\u003Coptgroup label=\u0022Backup_migrate\u0022\u003E\u003Coption value=\u0022backup_migrate|quick_backup\u0022\u003EBackup and Migrate Quick Backup\u003C\/option\u003E\u003C\/optgroup\u003E\u003Coptgroup label=\u0022Block\u0022\u003E\u003Coption value=\u0022block|30\u0022\u003EAccess Link\u003C\/option\u003E\u003Coption value=\u0022block|31\u0022\u003EAttempting to unsubscribe user account\u003C\/option\u003E\u003Coption value=\u0022block|29\u0022\u003EAvailable tokens for registration email\u003C\/option\u003E\u003Coption value=\u0022block|14\u0022\u003EDashboard Search\u003C\/option\u003E\u003Coption value=\u0022block|12\u0022\u003EDisclaimer\u003C\/option\u003E\u003Coption value=\u0022bloc

And the pop-up window just shows a rotating circle for a long time.