My site has just developed a serious problem after I tried to add a category on the 'Write Posts' Screen.
When I tried to add a category initially it says:
'That category name is invalid. Try something else.'
I then try something else and it creates about 5 new entries in my categories display all called 'undefined'.
After this has happened I couldn't access the Write Post screen or the Manage Categories screen again, I just get a blank screen and my browser says 'done' - no error messages or anything.
I then tried restoring my database from my local computer but this only fixes the problem until I try to add a category after which I get the exact same problem.
Any ideas? Its driving me mad and has made my site useless.
WP version - latest