I was having the same problem. In order to fix it go to Bepinex/config and find the file: com.sinai.BepInExConfigManager.cfgi kwon about informatics quite well, but i have only tried to make it work about 30 min this mod thing but unsaccfull, i had play blackmarkect with mods before,...
Open it in notepad and go to the bottom of the section titled ##The toggle for the Config Manager menu and set Main Menu Toggle = F5 to whatever you want to change it to. I changed it to Period and got it to work. I think F5 does something else on my comp and is interfering with bepinex. Hope that helps.