The file does not exist, you are not allowed, or is the file may be in use.

I can see my Library A-OK. I can double click to edit objects, and copy them from one document to another. But when I click to drag an object from the Library to a new document, occasionally I get a "The file does not exist, you are not allowed, or is the file may be in use." error. I know I'm logged in, the file exists and everything is OK. But my client doesn't understand and I need to reboot/login/logout/panic until it works again.