hmm, maybe you had it running twice or another AC server plugin was interfering, thus the error.
If you want to use several AC server plugins, you have to use AC_TrackCycle as UDP Broadcast relay,
using <add key="external_plugins" value="OtherPlugin, 11001, 127.0.0.1:12001"/>
I doubt that the router has anything to do with the problem.
Maybe try disabling the port on the router, then restart your PC and try again.
If it really doesn't work for you without forwarding the port, then I don't know why, sorry.
If you want to use several AC server plugins, you have to use AC_TrackCycle as UDP Broadcast relay,
using <add key="external_plugins" value="OtherPlugin, 11001, 127.0.0.1:12001"/>
I doubt that the router has anything to do with the problem.
Maybe try disabling the port on the router, then restart your PC and try again.
If it really doesn't work for you without forwarding the port, then I don't know why, sorry.