If it helps anyone:
the upgrade according to the above went without any big problems. The only problem was proftpd, where we see from bullseye does not support the IdentLookups configuration option, so proftpd remained unconfigured. The fix is trivial. Reboot Cloudmin and Virtualmin completely in progress with a new kernel on an encrypted file system. They both ran.
To start, webmin detected a distribution upgrade, so I offered to change the presentation from Debian 10 to 11. After acceptance, he changed it. So everything is great.
The another problem occurred with the Virtualmin installation. Apt offered me autoremove for bind, mariadb, postfix and many other packages that Virtualmin depends on. So the dependency chain was broken somewhere. How to fix it? With Apt reinstall Virtualmin? Or? It would be convenient to treat this with some Post-Invoke script. It could happen that someone will use the offered autoremove.
And another thing: somewhere in Cloudmin, the binding to the IP address of the xen guest guest with Virtualmin installation went wrong. Status of this host in Cloudmin is “Ping failed (Last changed at 29.04.2023 01:42)”. Detailed status error ssh: Could not resolve hostname 62.168.116.188/29: Name or service not known. I suspect that this host is presented with an IP address with postfix netmask format, i.e. XXX.XXX.XXX.XXX/YY. Other xen guests are presented only with IP in the classic format without the netmask postfix, i.e. XXX.XXX.XXX.XXX.
However, both pings and ssh logins to these machines from the Internet and between them work. What should I do about it? The network is configured correctly on both machines. This is how I can’t manage a Virtualmin machine from Cloudmin. I’m guessing it’s some glitch in Cloudmin, but I have no idea what to look for. After the dist-upgrade, I first restarted the Virtualmin host, and it worked perfectly. Virtualmin was on Debian 11 and Cloudmin was on Debian 11, but the kernel was from Debian 10, i.e. before restarting. After restarting the machine with Cloudmin this error appeared.
But otherwise: after our never ending problems with the previous webhosting panel we used, this was great. A nice job is done on this software(cloudmin and virtualmin).