Know a fix for the "Your attempt to edit this post ... has failed" error? How can I reins

Recently, I auto-upgraded to WPMU The upgrade occurred without error, but sometime afterwards I began getting errors almost everytime I try to save or publish a post. My recourse is to access the autosave of the post via edit and then publish it. Usually, on the second try it works. This is annoying and has actually reduced the amount of time I spend blogging considerably.

I did a little searching online and found that this can be caused by certain plugins. However, the error continues to occur even after I disable all my plugins. Some people have reported that upgrading their servers to php 5.x solves the problem, but my host assures me that the server is already upgraded to the latest version of php. Any clues as to how to fix this?

Here's the text of the error:

"Your attempt to edit this post "..." has failed"

Here's a blog post discussing the error:

Please help! Is there any easy and fast way to reinstall WPMU without screwing up my database and thus damaging my existing site? I've been posting on this site for approximately a year and have about a thousand posts.

Thank you!