[2020] Stellaris steam launch returns a bootstrap error

We now have reports of this on AVG, Avast and possibly Kaspersky.

You need to make sure all these apps are in its exceptions lists:

dowser.exe (in the Steam Stellaris folder)
C:/users//AppData/Local/Programs/Paradox Interactive/launcher-v2.2020.16/paradox launcher.exe
C:/users//AppData/Local/Programs/Paradox Interactive/bootstrapper-v2.exe

Good luck!  

That means your AV app has quarantined it. You need to release it from quarantine, and tell it to leave it alone in future!