Hello,
Is your issue resolved? Firstly, there are some good suggested solutions above, but I would like to fill in.
If it's not fixed, I would suggest as well as doing what's above you uninstall TruckersMP (C:\Program Files\TruckersMP Launcher), delete the folder, and verify the integrity of game files (Steam: Library > Rightclick ETS2 > Properties > Local Files > "Verify integrity of game files..."). Then install TruckersMP again, and log in then see if this worked.
If your issue is still going on, let us know in case someone has a different solution that might work.
Best Regards,
Axo