Thanks Adam.
Before I saw your last post I put a clean install on the server and that worked. So I pointed the clean, working config file to my database and the error started, so I knew it was a db problem. I copied the main cutechat tables and ran the db script over the db to get new tables. By doing this I was able to identify that the CuteChatUser table was the cause of the problem. I have no idea why or how as the database was moved whole from the (working) development server to the live server without any changes. Anyway, I wrote a script to pull the users from our main users table and insert them into the fresh cutechatusers table.

After that it all worked. Very odd, but I'm not going to worry too much about it.

Now, any chance of a reply to my other post about inviting users? ;)