Usually it appears when you try to update the multiplayer launcher with the game open.
First, check if ETS2 process is running. If so, end it and try again.
If the same error comes up again, please try to run the launcher as administrator.
If this doesn't help delete core_ets2mp.dll, re-open launcher and re-download file. Path: C:\ProgramData\TruckersMP.
Unreliable Connection kick mostly happens when your PC can't handle with so many players loading at once near you. To avoid that, you need to put lower graphics, avoid spawning in the crowded area and close all unnecessary processes like browser/Antivirus before you join the server.
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Expectancy's post in help with error was marked as the answer