Just had this happen to me.
For me. It was something that the game did not think was formatting right resetting my modsettings.lsx file. I found the file at *userpath*\AppData\Local\Larian Studios\Baldur's Gate 3\PlayerProfiles\Public. To see if it is the same thing for you. Save your order in the BG3ModManger, export to game, and then go look at that file with notepad or wordpad. Check to see if the mod order was placed in the file. If it has, launch the game and then exit when you get the mod error. Check to see if the modsettings.lsx file had the mod order reset to be blank. If so, then your game is not likeing one or more of the mods in your order when you are trying to load it and is resetting that file. Imoporting the modfixer mod into the bg3modmanager and moving it left worked for me. Make sure that just the .pak file ends up in your mods folder. If it wasn't the modfixer, I would try loading in the mods one at a time on a new game to see which one breaks it.