ProMods

to solve the problem with your game try to verify the integrity of your game cache
in steam right click on eurotruck simulator, properties, local files, verify integrity of game cache

for multiplayer you need to update to version 1.14.2.2s
in steam right click on eurotruck simulator, properties, beta's, choose version 1.14.2.2s in the drop down menu

Already checked integrity of game cache.
Should I downgrade it to a lower version? According to ets2mp.com I need to have the latest version.

Yeah for the current patch you need 1.15.x

Oh, they have updated it :p It's been a while since I played multiplayer. so yeah you need the last version.

I installed the latest multiplayer update. ETS2 is automatically updated by Steam.