Updated the guide with the automatic installer guide. The manual installation as well has been updated with new screenshots since the previous were outdated and client files have changed.
New procedure to fix "game won't start" error has been updated.
1) Game won't start
Try starting "ivmpGameLauncher.exe" with admin rights
Go into singeplayer, load a game and then quit. Go back to the launcher and the game should start (been tested before and proved to work)
If you get this error message: MSCVP140 is missing or error 0xc00007b
You either don't have the C++ 2017 redistributable or it's corrupted. Reinstall the x86 redistributable with the link provided here
6) Game crashes before loading (either at a black screen or doesn't launch at all)
1st fix: Download patch 1.1.3.0 and install it again.
If that doesn't work you can try to install the patch manually. In order to do that you have to rename your current "Grand Theft Auto IV: The Complete Edition" folder to something else (any name is fine), then make a new folder named "Grand Theft Auto IV: The Complete Edition" and install the patch again. Once it's done you will notice the patch files are now inside the new folder you created. Now, select all the files inside it and copy them over the original EFLC folder. Once you have done that you can delete that folder you made and rename the other one.
2nd fix: Rename the file "paul.dll" to "paulDISABLED.dll"
3rd fix: Download XLiveLess from GTAForums topic and follow the instructions over there.
Previously updated also other sections of Troubleshooting.
7) I'm running the game on 1.1.3.0 patch, how do i downgrade it to 1.1.2.0?
It is not mandatory to run 1.1.2.0 patch in order to play on the client anymore, but if you wish you can follow the steps that i wrote on this detailed downgrade guide for GTA IV that will work also for EFLC, click over here. The only difference is that you will also need to copy the files inside EFLC folder, but the procedure has not changed.
If you find any error or have any suggestion to improve this thread, please send me a forum PM.