After more try and error I sorted out what causes the kernel panic.
By default, mikrotik assigns a Link-Local IPv6 Address to the Wireguard Server Interface.
If you then assign a global IPv6 IP to that interface and a client is connecting the kernel panics and the Mikortik reboots.
Simple solution: Delete the LD IPv6 Address on the Wireguard Interface so there is only the global one.
That way it not works for me.
IPv6 routing via Web-GUI is broken. So if you want to add routes to an IPv6 network ‘behind’ a WG client, you have to do this via CLI.