Hi,
I am having this issue too. I've read all the tips mentioned in this group, and nothing works. It used to work before...
I am using GAM 3.45 on a Mac
I have generated a .json file, tried to replace it, checked the APIS in the console, disabled Admin SDK API ant re-enabled it, no succes.
I've read some things about creating a public api key for the browser or iOS, but I guess this is not applicable in my case.
I am not really a developper, so I don't really understand everything, I've carefully followed the instructions in the wiki, with success... What is odd to me is that it used to work !
Thanks for any help, I am about to finish my entire setup for my school. That's the last piece of setting that's missing !