Just a little update if someone reading this who has the same problem:
Reinstall Mobian and switching to Tow-Boot also does NOT solve this specific boot problem.
(One could hope...)
Really wasted 270€, shouldn't have been so over-enthusiastic and "buy a second PinePhone as reserve/backup, just in case" like I did...
(Irony: If the phone wouldn't boot at all, just never (not even after beeing off for a while), that would be better - Then I would have noticed the issue earlier and would have had a chance to sent it back to Pine64 and try to get the money back. Damn.)
Reinstall Mobian and switching to Tow-Boot also does NOT solve this specific boot problem.
(One could hope...)
Really wasted 270€, shouldn't have been so over-enthusiastic and "buy a second PinePhone as reserve/backup, just in case" like I did...
(Irony: If the phone wouldn't boot at all, just never (not even after beeing off for a while), that would be better - Then I would have noticed the issue earlier and would have had a chance to sent it back to Pine64 and try to get the money back. Damn.)
Using encrypted Mobian (bookworm) on pmOS CE Convergence PinePhone (3GB/32GB) as Daily Driver, and Beta Edition Convergence PinePhone (3GB/32GB) with encrypted Mobian (bookworm) as Backup device.