1), Do you use Steam? IF you haven't, you really should since ESF won't really work perfectly on WON: http://www.steampowered.com (i think you have it already due you see servers)
2), Have you installed ESF v1.2.3?, If you haven't, download it here: http://www.esforces.com/?p=files (you get a client error due you are attempting to connect to a server that has not the same version as you have installed, v1.2.3 is the latest version. Download it at the download page linked above)
WAIT A SECOND!!!!!!!!!!!!!!
If your'e going to install the patches (& jsut the patch installers not the full installers), your'e going to have to install them in order (1.2.1>1.2.2>1.2.3). Install 1.2.2 before you isntall 1.2.3, as 1.2.3 does not change the client dll, it's just a few map fixes & add a new map (esf_arrival).