Error when filling in the tags field - An AJAX HTTP error occurred.

I get the following error when I try to insert a tag and I also noticed this error occurs when I try to change the author of that page

An AJAX HTTP error occurred.
HTTP Result Code: 200
Debugging information follows.
Path: http://www.thesite.com/user/autocomplete
StatusText: OK
ResponseText:
[ ]

and

An AJAX HTTP error occurred.
HTTP Result Code: 200
Debugging information follows.
Path: http://www.thesite.com/taxonomy/autocomplete/field_field_tags
StatusText: OK
ResponseText:
[ ]

Nothing else is wrong with the site. Does anyone have any idea what's up here? I'm not even sure where to post this problem. Thank you.