If you want Krieg and UVHM you need to play on 1.8.3, 1.3.1 is only useful for all quests speedruns

Do you have the files that I need to play on 1.8.3?

You can find them in the resources section

My game doesn't start when using those 1.8.3 files, do you have a separate .exe I can use?

As far as I understand it, you need to patch from 1.8.4 to 1.1, then back from 1.1 to 1.8.3 for it to work. Is this what you did?

I wasn't sure if I did that so I repatched now with that method, still doesn't work. It just makes a STEAMSTART file in the win32 and then crashes. The launcher works but not the borderlands2.exe.

Not sure why it's not working for you, then. Not many people check these forums regularly, so I would recommend trying to ask in the discord server. It's likely that someone there will be able to help you with the patcher.

https://discord.gg/VZcd8K2