Usually it is "Connection Error" or "Speed Hack Detected Error"
Honestly i do not recommend this to fix it if you got important stuff on your computer, i use to get it after they updated and patched all of the "effect+area" folders where we could add our own .ssf files, allowing us to cham/add x100 bullets without having to inject a dll, so i basically uninstalled Soldier Front like 30times and nothing...But then what i did was a reset my computer to factory mode...this allowed me to play.
Sorry no German, use Google translator.