You have hit a new issue but without logs and evidences we are blind.
So…when you try to migrate everything is logged to journald in NS8
At the end
journalctl > dump
On ns7 you have a log in /var/log/
If you have the same issue like webtop for mariadb we stop at any error in the restoration of the dump so maybe a clue
I need to wake this up (and use it to push myself too). I will retry this weekend and try to supply the logs.
The whole thing is a bit… pathetic (the situation, not the project, I love the project)… I mean it is a simple setup, an NS7 setup that I just want the emails (and account provider) and I cannot do it.
The account provider has just 4-5 users, the mails were also migrated years ago to NS7 from PST files (plus of course all new mail received daily). NS7 managed to import, but NS8 has an issue to import something that is working in NS7 (which should be 100% success, it’s not that we try to bridge competing products or whatever… I mean I could understand failing to import some PST… but Linux mail subsystem to Linux mail subsystem?).
In logs we trust
Guys I cannot believe it…
IT WORKS! I MIGRATED FINALLY!
Something you fixed between early June and today fixed the issue.
No, I still couldn’t migrate WebTop. Failed the same way.
But I removed WebTop, migrated mail and account provider fine! No timeouts or anything!
I am at my NS8 cluster now, NS7 is shut down and mail seems to work fine.
I may or may not reinstall Webtop. NS8 needs some reading now. 
I am thinking of making a NethSecurity VM too, but to be honest I could love a complete DNS implementation in it (like Technitium) - still a caching DNS, no go.