Hello.
No, don't worry, it s the group id, this one is the invisible group used by the gateway, is a group will all your lights (used by phoscon, useless for domoticz)
The normal procedure is an error at start (because this group is invisible, so domoticz can't see it during synchronisation
2020-10-15 17:23:12.031 Error: (deCONZ) Websocket error, unknow device > 65520 (groups)
After that domoticz ask for information and add this group to domoticz, and avoid all new error message.
I think you have disabled the "new device addition" option in domoticz, so the plugin can't add it ?