Thank you all for the reply.
This topic was a bit overwhelming for me, as i had no idea what i am really doing.
After further digging, my understanding is that Thunderbird and davix5 does not use exchange active sync to retrieve the calendar and address book, but use SRV/TXT and “well-known URLs”.
The relevant info for this:
Davx5 - How does service discovery work:
https://manual.davx5.com/accounts_collections.html
Davx5 tested with SOGo
Based on this i assume that the well known URLs were configured properly on NS7 out of the box, but are not on NS8.
This can be verified in the logs with a bunch of 404 errors while running davix5 or Thunderbird account configuration.
That means, the solution for me was to enter the complete path into davx5 ( https://example.com/SOGo/dav ) to get it to sync.
In Thunderbird, once the mail account was added, i manually added address book and calendar using the Link from SOGo Web.
The best solution would be probably to configure the well known URLs, but i have no idea how.
I assume this would be done traefik?
Sidenote:
The NS8 Admin page does not contain any useful links regarding Integration via Caldav /Cardav/imap.
https://docs.nethserver.org/projects/ns8/en/latest/sogo.html#integration-via-caldav-cardav-imap
Davdroid link is not found and the other link ( CalDAV-Sync/CardDav-Sync ) just opens a blank webpage.