Add new post > “An unidentified error has occurred.”

Suddenly it has become a problem to add new posts to my blog. As I type my entry the message ‘An unidentified error has occurred’ appears just below the word-count text. When I go to post the topic it appears to be saving as a draft, but it just returns me to my page of currently posted entires, without adding my new submission. I’ve searched high & low for an answer on how to resolve this, but no such luck.

There are values entered in my custom field.
My permissions on my blog folder: User: 755. Group: 775. World: 775.
I am unable to disable plugins due to this error, but all are upto-date.
I was unable to automatically update my version of wordpress, so I had to do it manually.

I am desperate to fix this. Any suggestions please?