Autocomplete doesn't render when using @mentions

Autocomplete never renders, whether using [@uid], or any of the workarounds. From the investigation I've already done, I've collected this info:

1. User doesn't have a foreign keyboard
2. Encoding is UTF-8 and Wiki style renderer is enabled
3. All other methods of mentioning don't render user list
4. Found this error while looking through logs, but don't know what it might refer to.