Return Code 0 on New Blog Post or Edit

My blog has recently started alerting 'Return Code 0' whenever I try to post a new entry or edit an old one. I've submitted a ticket but out of interest in getting this solved quickly I'm posting here as well. My content authors are fuming, there are new blogs they want to post and cant, and modifications to existing blogs, which can't be modified. This has effectively broken my blog.

As far as I know, no development was done on the site and the content authors cannot remember doing anything out of the norm.

Whenever you click on 'Publish', the page shows 'Return Code 0' and does not allow you to proceed. 

Please see the attached file for the error returned from Chrome network diagnostics from the 'Inspect Element' screen.

Please help!

Dear Mark
It's allways helpfull if you post your version number you are on. Somestuff usually is fixed in later versions.
Markus

I'm running version 4.3. Thanks