Note, this isn't an in code problem, this is error when I as developer attempted to configure a service in a browser using Google APIs Explorer.
The service is Google Analytics Reports API v4 and when I go to toggle "Authorize requests using OAuth 2.0:" it throws a dialog to Select OAuth 2.0 scopes: where I check the checkbox for the readonly version 'https:://www.googleapis.com/auth/analytics.readonly' and the press the button marked Authorize. Whereupon a new dialog says "500. That's an error. There was an error, Please try again alter. That's all we know."
And I have tried again after an interval.
UPDATE: Tried again just now, still not working. Tried suggestion below. Also, tried on my IPad (as opposed to Windows 10 PC) to see if device specific.
I tried your link and it works for me, can you try and do it in an chrome incognito window with no previous cookies?
This looks like mixture some bad old cookies might be causing an error here