Track down error of terminated ajax request

I’m using forms with taxonomy terms and the autocomplete widget. On all forms – as soon as I start typing in that field – I get the error message:

An AJAX HTTP request terminated abnormally.
Debugging information follows.
Path: StatusText:
ReadyState: 4

And no suggestions for terms show up.
There are no errors showing up in the logs. (would I need an extension module for logging javascript errors?)
How can I continue tracking down the origin of this error?

Drupal version: