Strange user and error while deleting the file "there was a problem with the network"

Mijzelf said:

That user doesn't have shell access. No password, and no certificate.

You can try to find the package causing this:

cd /i-data/sysvol/.PKG/

grep -r "zK9UKHeN6" *

I found pairs of files in etc that differ by "-" (group and passwd and shadow) Should this be?

Yes, that is normal. It is the backup of the previous version created by adduser & friends.

The only thing I found is... I understand that this is just related to the cloud service.