It's ethier because one of you (the server you are tryying to connect to or on your side) are using different versions of esf. 1.2.3 Is the latest version of esf, though you should install 1.2.2 before you install 1.2.3 (if you already haven't installed 1.2.2) as 1.2.3 has no client dll changes (just map fixes & a new map, but you will still need 1.2.3 as maps will have changes to them and if you don't install 1.2.3 you can get steam telling you that "your map esf_"mapname" differs from the servers").