Uh, by the way, have you installed a new (but not the latest) hotfix? If not that could be the problem. I once had a strange problem while giving a special workshop for a customer. I wasn't able to fix it until I asked for the hotfixes, they hadn't installed them. After the installation and activation the error didn't occured again.
You shouldn't install the latest one, because some of the hotfixes (for 2007, 2010) were withdrawn by MS because the made everything worse