Exactly what it says on the tin. Starting the game via Vortex after the downgrade doesn't even bring up the start menu, it just fails. Starting it in Steam gets me as far as the start menu, but crashes when I click Start. Attempting to rerun the downloader produces the following error: "Error: Access to the path C:\windows\system32\error.txt is denied." Restarting my computer didn't correct it, and all files starting with CC have been deleted.
I am using mod organizer and its working perfectly. I think 1 of 2 things could be the issue: 1. there is something with vortex that is interfering with the downgrade or 2. your antivirus has blocked something with the downgrade(you can add an exclusion for it). Also when using mod organizer or any other mod manager you should be launching through F4SE not regular fallout 4. Lastly, launching through steam or launching through the fallout 4 itself will most likely either force update the game through steam or in other cases(if there is a problem with the files) it will crash or just not launch. I'd recommend going on steam and verifying your game files, create an exclusion for the downgrade files and/or turn off your antivirus, redownload the files, and then rerun the downgrade exe. see what it happens then. Also if you go into the "error.txt" file it will tell you exactly the problem(i had the same thing, i looked at the txt file and it said it was a problem with authentication which is most likely just the thing at the start where it asks you to authenticate your sign in)--this error is not a problem since my game is working fine.
I turned off my virus protector, created exclusions for the FO4 folder, Vortex mods folder and Vortex folder, then ran the downgrader again. I got the following not very informative error:
QR: False Authenticated: True Language: english Downgrade Creation Kit: False Downgrade All DLCs: False Delete Creation Club files: True Keep Depot: False
[Previous Errors] Error #1: STEAM GUARD! Please enter the auth code sent to the email at yahoo.com
Your Fallout 4 installation has been downgraded but with 1 error(s) and may not work as expected.
1 error on installation of mod, crashes on startup, can't rerun downgrader (6 comments)
QR: False
Authenticated: True
Language: english
Downgrade Creation Kit: False
Downgrade All DLCs: False
Delete Creation Club files: True
Keep Depot: False
[Previous Errors]
Error #1: STEAM GUARD! Please enter the auth code sent to the email at yahoo.com
Your Fallout 4 installation has been downgraded but with 1 error(s) and may not work as expected.