Unable to write errors are a common result of a full root filesystem. You may have too many saved snapshots and need to remove some older ones. SDB:BTRFS - openSUSE Wiki addresses this. If the root filesystem is old, you may have accumulated too many journal logs in /var/log/journal/<longlongstring>/ and need to use sudo journalctl --vacuum. See man journalctl. sudo systemctl edit systemd-journald may be used to configure various limits on journal size, and whether a persistent journal is even kept - /var/log/journal/ may not even exist.