Bug Unity Projects become broken

Hello. Can anyone explain to me how projects can become broken without any change?

Recently I've updated my Unity Editor to the 2020.3 version and converted a bunch of my old projects for this new version. After the conversion, everything seemed to work fine and I just closed everything. But later, when I've opened those projects (actually I only tried two of them so far but that's not the point) again, I had some error in the scripts out of nowhere.

I suspect that this is some sort of bug but I don't understand how is that even possible. Does Unity change something with every launch?

 

https://forum.unity.com/threads/uni...n-argumentnullexception.1089346/#post-7016206