Thank you very much for the thrust in the right direction. Now the stick is working.
I have Debian as OS. I had SUDO and a user installed befor deconz. This gave me a group problem, as I found out with your help.
tail -f /var/log/syslog -n100 | grep -i deconz
The query brought me the result:
Serial com disconnected, reasen: 4
Here i found the next tip.
Then I have
sudo gpasswd -a user dialout
sudo systemctl restart deconz
And look, the ConBee is working