It's probably a server issue, but there's a similar bug I've run into which might be happening in your situation. If the illustration has been imported into Quark, I get the errors you describe. Closing the Quark file with the imported Illustrator file doesn't clear it up, though. The only way to replace or delete (or save) the Illustrator file is to quit out of Quark. It's a lot easier than restarting the server, but it's still a bother.

This happened inconsistently, so I was never able to thoroughly trouble-shoot what was going on, since I couldn't duplicate the error at will.