>>107154343
Did a memtest, and it passed without errors. 'btrfs scrub' showed >100 errors,
and with dmesg I was able to get a list of corrupted files - thanks to btrfs! Without the checksums I would not know which files were affected, and I might not have noticed until all backups are corrupted, too... I might create a new btrfs on the disk and install a system, and check if journalctl still shows btrfs errors, if it does than I can be certain it's the hardware.
>>107155058
So it's almost certainly the ssd. At least now I know what to check regularly:
jornalctl for btrfs errors, and smartctl -a.

Thanks for the help everyone, I learned something out of this.