Can not save configuration form due to fatal error

This is a duplicate of https://www.drupal.org/node/2693499

Nobody is able to provide directions to reproduce the issue on a clean install or point to specific modules causing the issue. Nothing in the code refers to max_image_size_variable_settings_form_submit, so it is an incompatibility with a contrib module.

The patch provided more works around the issue by not figuring out why an additional submit handler is breaking and instead just putting the code elsewhere.

The bug is caused by the variable module

In variable_form_alter() the module alters the form submit function.
see: http://cgit.drupalcode.org/variable/tree/variable.module#n689

I am not sure if the variable_form_alter() function is correct, but changing the max_image_size_system_settings_form() function name (and its submit function) to something else should fix this issue.

Thanks Pasqualle, at first glance that does look like the culprit. I'll put some time on this a bit later to see if there's a better solution but if not I'll merge your patch.

RTBC. Please apply and create a new stable relase to fix the settings page. The patch is perfect.

I'm comfortable with renaming the function now that the root cause has been identified.

Thank you very much! :)