I wish I could remember exactly, but I think we extended the length of the GROUP DN earlier on with ARSLSYNC. Which is why upgrading worked.
"PS: I noticed, you need to run ARSLOAD twice if the group you are synchronizing does not already exist in OD." - I think you mean ARSLSYNC here.
As much as I want to pretend I didn't read this
.... That doesn't make any sense. ARSLYNC should create the users and populate them in a single run.