Charged Steam Wallet with 5 Euros - worked.
Tried Donation to Subsim again - and worked...!?!?!?
Yes, suddenly it worked, its still a THREE-factor verification, but this time the additional - accoustric - phone call actually arrived.
Neal, you now can buy yourself something to eat again, money's on the way.
Just don't ask questions. Dont' ask any questions on the How and Why. I have no clue.
No feedback from Paypal however. If they had done something, they really could have sent a brief mail to not let me in the dark that it is solved.