Thanks for pointing me in the right direction. The issue was in the RCON connection, which I overlooked because my loglevel was set too high.
My COD2 server bound to IP 0.0.0.0 instead of my WAN_IP. For some reason, B3 didn't like that and refused to connect over RCON....
I solved it by adding
Code:
 + net_ip WAN_IP
to the startup command of my server.
Now it works as intended, using IP instead of GUID

Thanks!!