@Tscheurgen Not 100% sure if I understand your issue right here, but it might be beneficial to take a log of about 5 mins length while trying to do whatever with your groups.
I recently also had a condition where anything group related just seemingly was non-existent to deconz. In my case, there were some 0xD2 errors stating the broadcast table would be full (absolutely not!! ).
In the end, it required ppowering down the system and removing and power from the coordinator device. So, if you’ve a Conbee I/II, remove the stick for some seconds. A device with Raspbee I/II needs to be cut from power.