Hmm.  A lot is going on during sign in. As a guess, a file is getting locked somehow.

Some clarifying questions.

• Does this go away for awhile if you restart your machine?
• Do you get the message after entering your user name/password and hitting "Sign In"? 
• Are you using Professional Network, Professional or Basic?

Also, can you take a screenshot of the issue, and email that and C:\ProWin20\32bit\logs\proseries.txt to proseries_engineering@intuit.com?