Hello,
I found an option to downgrade ETS2 and other games in Steam.
You need to open Console in Steam ( First activate it ). Then download the Files from an older Update. Then Copy then into Game Directory and override existing files. ( exe , dll. and log files ).
And then you have an older Version. I have tried it, and it works.
Here is the Guide trough it : ( This works with every game )
https://steamcommunity.com/sharedfiles/filedetails/?id=1086279994
A little help : This is the command to download the older Version :
download_depot 227300 227302 3938941869032024931
Edit : TruckerMP Update released a few minutes ago, downgrade not needed any more. But i hope this could help for next time, or other games with the same Version Problem.