WordPress Theme Editor Cannot Be edited

Hi!

I’ve recently upgraded to 2.9.2 and after deleting some test posts and adding a new one, I went to edit my theme, and got this message:

Sorry, that file cannot be edited.

I’ve tried changing themes to the standard and deactivating some plugins, but it doesn’t seem to help. I even asked my web host and they say the problem is with wordpress.

Can anyone help?

Thank you!