Had the same problem few days ago after I have updated to the last version of ets2mp. My system is same x64 Windows.
For me, the solution was, and still is: patience for 2,3 minutes and it will start.
Practically, when I tried to lunch ETS2MP, nothing happened, only the process in task manager. So I closed it from task manager and retried with the same behavior (game did not started, only the process.). I tried the singleplayer version and it was ok.
Only by mistake, I found out that if I wait for 2,3 min, the games starts. Be sure not to have opened multiple ets2mp processes. Start it only once and wait, and wait.
It is still doing this, for me, now at this moment. I think it has something to do with the last update vs. my pc configuration.
Please tell me if it is the same case for you. Hope it helps.
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SisQuo11's post in Only process start was marked as the answer