Sorry to duplicate, but I have Gold membership on this and thought I won't get any help on other account.
So for two cars I happend to get exact 64000 XP on car level 4, which is supposed to elevate to level 5.
It says level 5 on the cars - but screen after games are stuck on car level 4 and 64000 XP, 0 XP left - and never updates from there.
Two cars blocked from making further progress.
I have Ultimate Collection Forza 3, as well as Forza 3 bundled with console, both installed on HDD.