Hmmm, normally I read FAQ's.
From the FAQ:
First of all, make sure you have register_globals=on in your php.ini file (if not: change it to on, restart your webserver and try again).
The next thing to check is that you have file_uploads=on in your php.ini (again: if it isn't, change it to on, restart your webserver and try again).
And if that didn't fix it, then you're experiencing a still unresolved problem in your server configuration (there is some indication that this might even be a bug in PHP itself). The only known workaround at this point is to remove the enctype="multipart/form-data" from the template file for the Admin story editor (layout/YourTheme/admin/story/storeditor.thtml). You will lose the ability to upload images in stories after that change, but you should then be able to create and edit stories.
I now recall that I was only having problems with stories that had images so maybe the multipart/form-data was biting me, as I have register_globals and file_uploads enabled.
However, I can now edit stories, but not topics. Strange, guess more investigation is in order.