Diagnosis: Client machine states that they are out of disk space or memory when opening attachments or documents from Outlook.
Solution: (Depending on document type) Open Excel, Word, Powerpoint, Access, whatever and go to File -> Options -> Trust Center, then open the Trust Center and go to the Protected View section, and uncheck all 3 boxes from within.
I don’t know if there’s a better way to do this, but this solves the problem for all machines I’ve done this to.