Deconz headless service code=exited, status=1/FAILURE

Dear community,

I am running a raspi 4B with raspian (based on debian trixie) with iobroker and deCONZ/phoscon in headless mode.

Since a couple of days the deCONZ service keeps crashing a start and restarting.

systemctl status:

* deconz.service - deCONZ: ZigBee gateway -- REST API
     Loaded: loaded (/usr/lib/systemd/system/deconz.service; enabled; preset: enabled)
     Active: activating (auto-restart) (Result: exit-code) since Wed 2026-11-04 18:29:28 CET; 13s ago
 Invocation: d4b06899756e4f7b8d3c353b00536db8
    Process: 2675 ExecStart=/usr/bin/deCONZ --dbg-info=2 --dbg-zdp=1 --dbg-zcl=1 --db-aps=1 --dbg-http=1 -platform minimal --http-port=80 --ws-port=441 (code=exited, status=1/FAILURE)
   Main PID: 2675 (code=exited, status=1/FAILURE)
        CPU: 157ms

The service deconz-gui is disabled as this is a headless setup.

As you can see in the status → ExecStart I already enabled debug output but it outputs only a qtk warning:

Detected locale "de_DE@euro" with character encoding "ISO-8859-15", which is not UTF-8.
Qt depends on a UTF-8 locale, and has switched to "C.UTF-8" instead.
If this causes problems, reconfigure your locale. See the locale(1) manual
for more information.

(deCONZ:2697): Gtk-WARNING **: 18:33:00.294: cannot open display:

I ignored the charset output for now as it currently seemed not relevant.

Is there anything else I can look up tracing this down?

Thanks in advance.
Michael