My backup of / (I excluded /proc/* /sys/* /tmp/* and /var/*) had one error:
error: read /run/snapd/ns/lxd.mnt: invalid argument
What does that mean and will this make a disaster recovery impossible?
/run is a temporary filesystem which can lead to Such errors. You might want to exclude it as well. Or maybe you want to take a look at the --one-file-system Option.