If you’re 100% sure about you’ve written the email address and password without any mistakes, please try the following:
1. First, open up a text editor of your choice (e.g. notepad, notepad++,...).
2. Then, write down the email address as well as the password.
3. After that, copy them (CTRL + C) one-by-one and paste in-game with using the CTRL + V combination.
It’s very likely that you can log in without any issues now.
If you're still facing the problem (this solution usually works!):
If you're still facing the issue, you should try to delete the Steam_api.dll file as it might be corrupted. To do so, please follow these instructions:
1. First, head over to the installation folder of Euro Truck Simulator 2 (or American Truck Simulator).
If you don't know how to get to the installation folder of the game on Steam, read the instructions in the following spoiler:
Reveal hidden contents
2. Once you’re in the folder where your game files are located in, please delete the folder called "bin".
3. After that, verify the game files.
if that don't work do a support ticket