If this is showing up in ALL places you create, you most likely have a backdoored plugin. I suggest going through every plugin you have and checking if they are the legit version or not.
Are you sure? I would check your plugins in studio just in case and disable/remove any. You may have installed one and forgotten about it.
To check for backdoored scripts in studio, you can also press Ctrl+Shift+F to search every script in your game. The two lines of code you are looking for are getfenv or require, so search those and delete any unwanted results.
Honestly I have no clue if it’s not that, the only other thing I can think of is if you have modified your Roblox Studio files? Maybe someone else can help out.
I also just wiped my disk and reinstalled my os. And the error is still happening. I tried switching my account and that gets rid of it. So I guess its some kind of plugin but I don’t know what to find it. I looked everywhere but I can’t find anything.
Call your computer company or something like that or microsoft your pc might have a virus in it you should do it as soon as possible before it gets worse.
And also is your studio glitching or lagging or something like that or is just poping up only on the output?
I had a 403 error for a long time but I never thought anything of it. Today I loaded up a project and this replaced the 403 error. Also I checked its not just that one project its even brand new ones.