I succeeded in starting domoticz again by deleting the device in the domoticz.db.
I tried to add another switch with the green arrow from the devices table (another bulb) and when I change the status of the switch (dim or on/off), domoticz crashed again with seg fault.
I really don't understand ...

Hi moroen,
I had the same error than andi2016 :
python3 tradfri.tac produces
for group in self.groups:
builtins.TypeError: 'Group' object is not iterable

Disconnected

coap-client -k xxx coaps://192.168.1.108/15004/
returns
v:1 t:CON c:GET i:c822 {} [ ]
[159174]

and
coap-client -k xxx coaps ...