VC-3.1 500 - belt jumping out of flange

Hi guys,
I have upgraded from VC-3.0 to 3.1 and now it seems that very soon after a print start (and also manual movement) the bottom belt is jaming as it‘s moving out of the corner flange.

Is there any trick?

It seens like it‘s only the right side on the motor block but everything is set correctly:
- All precisions shims
- Belt tightening (tried very tight and not tight)