eureka !! I solved it.
This is how:
I removed the "Game data" file on the PS3 (less then 100k). Then I started the game and went straight to the 'in Game store' and to the Mappack. (that was not possible before when I had the SeasonPass installed... But now it show the mappack and I didnot get that ERROR msg..
Notice: (Do NOT download the seasonpass again, before you have downloaded the mappack... Maybe you dont have to download it again ?).
voilá.. the Onslaought mappack (about 1182 MB) appered and I could download it..
:-)