installing a new patch changes your version of the game.. the server you try connecting to needs to be running the same version.
Note- if its specifically a multiplayer patch... thy sometimes help improving some bugs or something, but thy dont always come out better, lots of times the older versions of games have better multiplayer features. i think most of the patchs are made just to try and make it harder for ppl to cheat. witch i feel is some what nessasary but stupid at the same time, cause there sacrificing gameplay to do so.