I just restored the old buster image and everything is working. I can pair new devices, can switch lights and get sensor data. (cloned the sd-card before the update)
pi@iobroker:~ $ lsusb
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 019: ID 1cf1:0030 Dresden Elektronik ZigBee gateway [ConBee II]
Bus 001 Device 002: ID 2109:3431 VIA Labs, Inc. Hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
pi@iobroker:~ $ groups pi
pi : pi adm dialout cdrom sudo audio video plugdev games users input netdev spi i2c gpio iobroker
pi@iobroker:~ $ ls -l /dev/ttyACM*
crw-rw---- 1 root dialout 166, 0 Dec 14 17:15 /dev/ttyACM0
What can I do to fix the bullseye installation?