I had the same problems.
Regarding those 2 errors in backoffice, go to your modules and check Google Analytics API because after that fixing it came again like "Install". I clicked on it, and it was written that it is already installed but errors disappeared.
Then regarding "Cannot retrieve test results" and this error:
Error: redirect_uri_mismatch
Application: TPV Analytics API
The redirect URI in the request: http:/..... /modules/gapi/oauth2callback.php did not match a registered redirect URI...
I have found that google sometimes cannot read "www" and really this worked for me:
1) In Developers Console - Consent Screen in Homepage URL - remove "www" so it will stay only http://domain.com and
2) in Developers Console - Credentials - Cliend ID - Authorized Javascript Origins
http://www.domain.com
http://domain.com
3) and in Authorized redirect URI
http://www.domain.com/modules/gapi/oauth2callback.php