FYI, this happened to me yesterday for the first time in the seven years I have been using ProSeries.  Was freaked out at first, especially with the deadline coming up next Monday.

Trying to log back in several times does not help, it just starts the 15 minute clock over.  What does work is running the Repair Update on the particular year you are working as described above, then restarting your computer.  That did the trick for me and left me very relieved.