Disconnected from Home Assistant

Since yesterday, I have a strang issue which I am not able to solve. I am using AppDaemon to control a couple of Hue dimmers which worked perfectly until yesterday. But now AppDaemon refrains from connecting to HA itself:

2024-12-22 11:28:04.934840 INFO HASS: Connected to Home Assistant 2024.12.5
2024-12-22 11:28:05.035304 INFO HASS: Evaluating startup conditions
2024-12-22 11:28:05.049316 INFO HASS: Startup condition met: hass state=RUNNING
2024-12-22 11:28:05.049929 INFO HASS: All startup conditions met
2024-12-22 11:28:05.084865 WARNING HASS: Error getting Home Assistant state for None
2024-12-22 11:28:05.086179 WARNING HASS: Code: 500, error: 500 Internal Server Error
Server got itself in trouble
2024-12-22 11:28:05.087582 WARNING HASS: Disconnected from Home Assistant, retrying in 5 seconds
2024-12-22 11:28:10.103839 INFO HASS: Connected to Home Assistant 2024.12.5
2024-12-22 11:28:10.240978 INFO HASS: Evaluating startup conditions
2024-12-22 11:28:10.261306 INFO HASS: Startup condition met: hass state=RUNNING
2024-12-22 11:28:10.262289 INFO HASS: All startup conditions met
2024-12-22 11:28:10.294475 WARNING HASS: Error getting Home Assistant state for None
2024-12-22 11:28:10.297821 WARNING HASS: Code: 500, error: 500 Internal Server Error
Server got itself in trouble
2024-12-22 11:28:10.298996 WARNING HASS: Disconnected from Home Assistant, retrying in 5 seconds

This repeats over and over again every 5 seconds. Several HA restarts (including RPi reboot) did not help. All my AppDaemon apps will be loaded until those warnings occur. I can see all the deconz_event events from the dimmers in the Developer Tools so technically there is still a connection of the devices to the system.

The only change I applied yesterday was the addition of a smart bulb (manufacturer MLI, model “ZBT-ColorTemperature”). It is connected via DeCONZ and is being successfully controlled via listening to a deconz_event event, but there should be no connection to what AppDaemon does (I think). There was no change in the AppDaemon setup at all.

What can I do to track this issue down, please?