@lxr : Thank you very much, that helped a lot.
I don't see the point in TurnkeyFileserver anymore, esp. as Turnkeys recommendation is to install a new LXC for each Upgrade. I tried a distro upgrade the regular debian way (not recommended, but...
Thanks for this. Unfortunately in TKFS16 Webmin does not provide any backup options, only via TKLBAM. So I am afraid I have to backup all settings (samba, nfs,rsync) manually. And I don't know, where these settings are stored...