Hello @JesperDen
First, check in the task manager whether you have an ETS2 process open. If so, finish it and try again. If the problem persists, delete the core_ets2mp.dll file located here: C \ ProgramData \ TruckersMP, run the Launcher and download this file again. Then check if the problem persists.
Kind regards,
xxTH3 B3STxx