I've been getting this error randomly for years--definitely back to at least 7, maybe even 6. It's very sporadic for me. Doesn't happen often, but when it does, I usually have to do a "Save as..." and then close the project. Reopen the project, and the problem is gone. This has happened on all my machines and all my drives for years, but again, for me, it's quite rare. Just happened yesterday, in fact, but hadn't happened for some months before that.