Hi Josh,
Thanks for your help.
I created new testing account
(541-978-5959) regenerated my refresh token but error still remains.
I think problem may be in Google Advertising Program Terms (in live MCC
818-778-6277) not being properly accepted. I've noticed that last night.
This is what I see in my MCC in Preferences:

I'm not sure how to approve it as there is no link/button to do so.
Any idea how can I approve it? I tried changing my CC info in billing but it didn't help.
Thanks
Darek