For me I found that the problem is when you have MORE THAN ONE GOOGLE ACCOUNT OPEN.

So a workarround is to open a private page or to use another browser, where you will only log in with one user account... In my case, I use firefox, so I do use it in Chromium where I just log with one of my google user accounts...

I also have issues when oppening app scripts from the menu when having more than one account open, so that is the root of the problem... google is not managing correctly having multiple google accounts open..

It says "Script could not be found" but internally it looks it is trying to execute it with the wrong account and then it is a permissions issue, so best is to have only one google account open and it does work as a charm..

Hope this helps everyone ;)

Pere - increiblesostenible.org