Hi Guys - it’s solved. Channels are closed and Funds are returned.
My main Error was that i forgot that Umbrel is running inside of Docker. I copied my channel.backup to my home-dir and tried to reference it there with a "./bin/lncli restorechanbackup --multi_file /home/user/Downloads ..." command. This throws no error but simply does not work.
So if you have an up to date channel.backup file, copy it to your /lnd/ directory which is inside the docker container. then run
cd $wherever_you_installed_umbrel
docker-compose exec lnd lncli restorechanbackup --multi_file /data/.lnd/channel.backup
as mentioned in this Post: Help Recovering Lightning Channel (Linux)
If you do not have the latest channel.backup File, you can contact someone of the Umbrel Team in Telegram and ask for help. Thank you @lukechilds for walking me through. He said that in the future, the Channel Backup File will be recovered automagically in the Restore-from-Seedwords-Process.