Dear @speedy4184,
If core_ets2mp.dll is not working for whatever reason you can get the above error. Usually it appears when you try to update the multiplayer launcher with the game open. Solution:First, check if ETS2 process is running. If so, end it and try again. If this doesn't help delete core_ets2mp.dll, re-open launcher and re-download file. Path: C:\ProgramData\TruckersMP
Anyway, the issue should be resolved since launcher is live again.