Thank you for the additional hints @GENLTD !
I am back from a vaccation abroad and after returning i did a new attempt on that topic.
1st step: i did the backup of the virtual servers after closing the external connections to them so that noone is able to alter any data on them.
Meanwhile the Promox PVE got another update from 8.2.1 to version 8.2.2 and my hope was that the migration process would now working better.
2nd step: attach the ESXi datastore to my Proxmox Environment
3rd step: Do the migration task
4th step: Boot the migrated virtualmin server and have a look what the interface name of the network card is. It has been changed to “ens18”
5th step: with nano /etc/network/interfaces => there was “ens33” configured and i changed all the ens33 entries to “ens18”
6th step: i restatred the network with systemctl restart NetworkManager.service and check the change
7th step rebooted the virtualmin server and enjoyed a fully working migrated virtualmin server!