Thank you for the suggestions. I already knew the Belt twisted twice discussion, the others are new to me.
I just checked the Part 338 and it is damaged - the ring broke from the core. So, probably this can be at least part of the problem.
So, now it seems that I have to replace at least the belt, the Part 338 and maybe the rear pulley as well? Where can I get all these spare parts? Bill, do you supply all spare parts?