Hi Toni!
Why don’t you install a new server (VM) with new debian,
install zammad in the same version as on the old system (but don’t configure it).
Then you create a backup of the old system,
copy it to the new server,
turn off the old server
configure the new server with the same IP-address and hostname as the old server and
then restore the zammad backup,
and if everything works, then update the zammad version from 6.1 to 6.2
maybe you have to run
zammad run rake searchindex:rebuild
after the restore