I have no idea if this will help anyone, but I got mine working.
Firstly I tried a different browser (safari) the authorise button worked, I received a code to copy and paste – this is not a UA- code as I thought, it’s quite a long format code alpha/numeric.
That code didn’t work.
I have 2 email addresses authorised as manager logins set up for Googles webmaster and Console tools to manage the 1 site.
I tried authenticating again, using the 2nd email address, and everything worked (despite both email addresses having the manager level).
I’m unsure where to find the Google access code manually to post a URL here, I just got lucky trying the other email.
I was using Firefox before. If anyone has knowledge on finding that Google authorisation code URL, I think that would be really helpful.
Good luck ??