That's the ajax JSON response. Looks like something maybe altering the output and it can't parse the JSON. What type of field is that and what value are you supplying to it? Do all fields with ajax enabled do this or just that field? Based off your description something is chopping off the closing quote and bracket for the JSON response for some reason.

My guess is something is installed altering the data buffer and breaking the JSON response or something jQuery related is breaking the ajax response. Try switching to default Joomla protostar template and see if it's the Joomla template (templates can override endpoints). If issue persists see if you've any header compression extensions (e.g. jQuery and CSS compression extensions) and try disabling them. If issue persists ensure CBs jQuery is enabled in CB > Configuration > Integrations so CB won't use Joomlas or whatever other jQuery is available. If still not working best I can suggest is disable 3rd party System plugins within Extensions > Plugins to see if any could be conflicting.