Why do I get an "error processing the request" message?

[Dan Moran]

When trying to publish a page that contains changes made by another user, I receive a Publish the page? pop-up that contains the following message:

You'll also be publishing changes made by these people:

<name>

When I click the Publish button, I get the following message:

There was an error processing the request.

Any ideas? 

Thanks, Dan