Was playing this morning with no issues.
Left game on whilst had a nap.
Session expired, logged back in.
Now when I click 'launch game' it freezes and I have to ALT + CTRL + DEL > [change user] to get my computer back.
Tried six times with three different maps, identical results.
EDIT: Came back after 8 hours and tried again: no problems. So, don't know what the issue was, don't know what resolved it.
EDIT: Two days later, it's doing it again.