I currently have the More Pedestrians and Traffic mod installed using the GTA V Mod Manager, but every time the I start the game with the modded rpf enabled I get this error: 'Game memory error. Please reboot and restart the game.' I've tried setting everything related to not read-only and starting both OpenIV and the mod manager as administrator and nothing works. As soon as I stop using the modded RPF the game works fine. Any idea what could be causing this and how to fix it?

Try doing it in a mods folder in the normal directory without GTA V Mod Manager and make sure you have OpenIV.asi and an asi loader installed too.