What causes 'Invalid form POST data' when removing an attached file from a node? I have an attached PDF file on a file field. When I click the 'Remove' button, the AJAX throbber appears and then the file remains unremoved. Adding an file to a new node still works.

I checked the error logs; it states "Invalid form POST data."

What is causing this problem?
I can confirm, increasing 'max_allowed_packet' in our MySQL config solved this problem. MySQL's default of 1 megabyte was too small. Increasing it to 16M, then restarting MySQL has resolved this issue.

Info on our solution was found here:
https://www.shift8web.ca/blog/2014/11/drupal-problem-an-ajax-http-request-terminated-abnormally/