In order to verify your game cache, go to your 'Library' and find 'Euro Truck Simulator 2'.
Once you find it, 'Right Click' and go to the 'Properties ' tab.
This will open up the properties tab. Go to the third sub-tab that says 'Local Files' where you will be presented with the following options.
Click on the last option to begin the validation process. This might take a few minutes.
Once all the files are validated, if any corrupt instances are found, then Steam will redownload those particular files for you. If not you will be greeted with a box that says no errors.
You have verified your game cache successfully.
1. Click Start -> Run (or WIN+R)
2. Type regedit and hit enter
3. Follow the path HKEY_LOCAL_MACHINE\SOFTWARE\TruckersMP
4. Look at the value parameter "InstallLocationETS2"
If they differ:
1. Copy the folder address from STEP 1
2. Change the value parameter "InstallLocationETS2" (Insert the copied folder address)
3. Click OK.
After that, many users get another error: "Connection error - retry?" In this case, open Task Manager, close all Programmes of TruckersMP/ETS and try to start the Launcher as an administrator again..
Disable the firewall and antivirus temporarily. If it works with them I suggest to change the antivirus