After downloading the game you need to run the UpdaterEx.exe to get the latest version of all files, otherwise it might not work at all and just crash.
If you already got that file named UpdaterEx.exe in your FOnline directory - eg. you just downloaded the last client that already does include it - you don't need to download it seperately or again, just run it (doubleclick it) before you start the game.
If you have problems with updating, eg. from a client of an older season, or your updater doesn't work for any reason, thank Slim who wrote this thing:
The old updater doesn't work. You need UpdaterEx.exe which is provided with the client since a long time. The fastest way to use new updater if you don't have it is:
1) Download UpdaterEx.exe and place it in game directory
2) Open FOnline.cfg with notepad and add these two lines at the end, then run updater:
NOTE: Backup the FOnline.cfg before you run your new updater if you want to keep your config.