Okay, it just happend again.
But this time, it happend on the other backup storage.
Backup logs:
INFO: 53% (583.6 GiB of 1.1 TiB) in 7h 2m 56s, read: 5.0 MiB/s, write: 4.1 MiB/s
ERROR: backup write data failed: command error: write_data upload error: broken pipe
INFO: aborting backup job...