Ah ah, yes tracing & debugging is what I've been doing and I'll keep doing.
It's just that after a while I also opted to share and increase the chances to reach a solution.
I've read this when Sebastian Riedel gave up with Mango develop
and I also know that Mongo no more officially supports Perl.
I had a chance to guess about the 'sharp tongue' reading both the discussion on issue 1223 and the following PR's review, but that is fine.

Yes, the versions of the old MongoDB cluster and the new Atlas one are different. It does seem, as you suggest, there is some incompatibility server side to also figure out.

Thanks for the advises and for your time.