Ye, i dont understand too...., I have P&G and the 2.1 patch, Dunno why i'm being kiked...
@found solution
how to fix it but it has to be on the servers side.
The server must go into GTR2\user data\"user name"\"user name".PLR ( in this case should be in the P&G folder )
open the PLR file with notepad and go down to the Multiplayer options part and find "report Mismatches" it should be set on 2 which is auto kick. change the value to 0 for No checking, and voila. It works. dunno how i found this I just did and it works every time for us. We play on LAN with tons of mods. F3000 works addon tracks work ....good luck