We had this problem last year and I posted then that the only user with the problem did Windows updates, which fixed the problem.  About a month ago, a similar problem reoccurred with QB/Outlook with only one user.  She forced her Windows updates (which she was behind on)...and the QB issue was fixed again.  I have a lot of unresolved issues with Quickbooks that madden me.  So, I am not defending them.  But in our case, this was a Microsoft issue, which Windows updates fixed.