[2021] Game suddenly cannot find my mods folder and won't let me get achivements even though i have no mods active.

What is your antivirus? Add CK3.exe to the exceptions list of your antivirus app; ESPECIALLY if you have Windows Defender, add it to the Ransomware "Apps Allowed Through" list.

You may need to add steam.exe and C:/users//AppData/Local/Programs/Paradox Interactive/launcher-v2.2020.16/paradox launcher.exe to that list.  

Did you also add the launcher to the Avast exceptions list?