After some analysis I think I got to the root of the problem, at least for me:
I tried everything before, even new backups an a new NAS where failing after a couple of minutes.
Now obviously the internal firewall dropped the SMB connection sporadically, which caused all the troubles and took me hours to get to (I have an issues with my tb4 dock as well at the same time, so it was quite a journey).
Solution now (for me):
- turn off internal fw
- start backup
- after couple of minutes, I switched fw back on (maybe should skip this one until first backup is done...)
right now, it's at 26GB of 1,1TB, running fine OTA.
edit: it stopped again, BUT after disabling the fw again it seems to run smoothly again...
update: still running, even with (macbook pro 2021) lid closed, on battery.
Maybe (!) it's a fw issues as well when going to sleep mode, as this might change something in the fw settings we dont see.