Both ends are running Webinoly 1.15. The old one throws a lot of errors on 'webinoly -verify' though. But the sites run.
I also manually checked the wp-config file for site 2 on both ends and the credentials were identical.
A difference between the two servers are that the old one runs MariaDB 10.2 while the newly installed one is on 10.6.
The reason for abandoning the old server is that I failed to upgrade it several times. Both from Ubuntu 18.04 to 20.04 but it also got totally corrupt when trying to upgrade php from 7.2 to 7.4 within Webinoly earlier this year (see separate thread). That's why I'm not even trying to change anything on the old server.
It is very likely that the nginx on the old server is corrupt while it still serves the WP sites correctly. I was about to fix it manually but you explicitly told me not to.
Log from second try on site 2 (overwriting):
Site nevergreens.se already exists... Overwriting...
Webinoly Site Backup nevergreens.se imported successfully!