Hello Markus,
I hope you are ok - I am using your library (mainly WebSocketServer) on my project for years now, and it is working well on ESP8266, ESP8285, ESP32, ESP32-Pico,ESP32-S2, ESP32-S3, ESP32C3, great library.
Unfortunatly the WebSocketServer crash on ESP32-C6, I am just starting the porting.

I digged in the issue using your example: https://github.com/Links2004/arduinoWebSockets/blob/master/examples/esp32/WebSocketServer/WebSocketServer.ino
and found that line is the root cause on C6:
https://github.com/Links2004/arduinoWebSockets/blob/master/src/WebSocketsServer.cpp#L92

it seems ESP32-C6 really does not like :

#define DR_REG_RNG_BASE 0x3ff75144
    randomSeed(READ_PERI_REG(DR_REG_RNG_BASE));

replacing it by the default :

randomSeed(millis());

Solve the crash issue, I did not do a PR because I am not sure this is the solution you want, or you want to screen C6 only for it, etc...
So I just share my finding - hope it help