The GMail Account went offline. It didn't even help purging everything from GMail to configure a new account.
Finally I found a solution here:
https://bugs.kde.org/show_bug.cgi?id=410872
Comment #7
> Open KWalletManager > Search for LibKGAPI > Maps > *usercontent.com. Remove all entries from there.
That worked for me! After that I got the workflow to authenticate my GMail account.
Regards, Bequimão