Ok, guys... As Patrick Oberli stated before (thanks, Patrick!), now I think we really isolated the culprit.
Actually the problem surfaced two times for me, even after the latest updates and all.
And what made it work again (without the need to logoff/login) was to CLOSE STEAM. Yep, it seems Steam is the great evil boss here. :P
Edit: I was thinking here... if it still doesn't work, maybe it's a combination of the new Creator's update and closing Steam. Worth a try, if you ask me...
Ok, this solved it for me. Closed steam, made sure task was dead, worked in Forza 3.
Wow