The weak key error isn’t something I can fix. You’ve got an old install. You can switch to the new(er) repository instead, which uses a newer key. You’d need to fetch the new key, and import it with apt-key import. Then switch the repo URLs to:
deb http://software.virtualmin.com/vm/6/gpl/apt virtualmin-buster main
deb http://software.virtualmin.com/vm/6/gpl/apt virtualmin-universal main
I’m looking into the other issue. Something broke in the mirror sync process and it’s not syncing anymore, for some reason. I’ll take the misbehaving server out of DNS until I sort it out, so in a few minutes you should be able to apt-get clean and try again.