This Crash is caused because of the Schwarzmüller Trailer Pack DLC its not supported yet to be played in MP, if you have this DLC, you have to Uninstall it for the time been.
here is how to do that :
1. Go to Steam > Libraray > Games > Right click Euro Truck Simulator 2 > Go to Properties > Go to the DLC Tab > Search For the Trailer DLC > uncheck it.
Then you have to wait tell the Bug get fixed, Hope this was helpful.