Google's Blogger V3 api oAuth not working anymore

I have been using the Blogger V3 api for many years. I use the official googleapis.com PHP library to retreive data and take care of the oAUth2 flow. Since a couple of days I cannot let users authorize my app anymore.

I did a lot of debugging and reading on the internet and here are my findings:

• The access_token that I receive after a user has authorized us through the oAuth permission screen, is valid. I can use the Google library to fetch the userinformation with it.

$oauth = new Google_Service_Oauth2($client); 
$userinfo = $oauth->userinfo->get();    

• The access_token is validated fine when using https://oauth2.googleapis.com/tokeninfo?access_token={token} . I receive a full object as should:

stdClass#1
(
    [access_token] => '{TOKEN}'
    [expires_in] => 3599
    [refresh_token] => '{REFRESH_TOKEN}'
    [scope] => 'https://www.googleapis.com/auth/blogger https://www.googleapis.com/auth/userinfo.profile'
    [token_type] => 'Bearer'
    [id_token] => '{ID_TOKEN}'
)

I can perfectly fetch Blogger information and content in the Google Playground when I enter the access_token that I have retrieved using the OAuth flow on my server. So the access_token seems to work within the Google environment itself.

However, when I try to request the Blogger V3 API from my server, I keep bumping into an 403 error. For example:

Your client does not have permission to get URL /v3/blogs/byurl from this server. That's all we know.

Here is my (Yii 1) PHP code for the oAuth2 flow:

Start authentication process:

$client = new \Google_Client();
        $client->setClientId({ID});
        $client->setClientSecret({SECRET});
        $client->setRedirectUri({CALLBACK_URL});
        $client->setAccessType('offline');
        $client->setPrompt('consent'); 
        $client->addScope([
            'https://www.googleapis.com/auth/blogger',
            'https://www.googleapis.com/auth/userinfo.profile',
        ]);

        $authUrl = $client->createAuthUrl();
        $this->redirect($authUrl);

Callback handling:

if (!isset($_GET['code'])) {
            throw new CHttpException(400, 'No code provided');
        }


        $client->setClientId({ID});
        $client->setClientSecret({SECRET});
        $client->setRedirectUri({CALLBACK_URL});
        $client->setAccessType('offline');

        $token = $client->fetchAccessTokenWithAuthCode($_GET['code']);

        if (isset($token['error'])) {
            throw new CHttpException(400, 'Google auth error: ' . $token['error_description']);
        }

        // For test purposes the token is just saved to session instead of DB
        Yii::app()->session['google_token'] = $token;

  
        $client->setAccessToken($token);

        $oauth = new Google_Service_Oauth2($client);

        // This works. An object with the user information is returned
        $userinfo = $oauth->userinfo->get();

 
        $service = new Google_Service_Blogger($client);
  
        // this doesn't work and returns the 403 error
        $blogs = $service->blogs->getByUrl([
            'url'=> '{BLOG_URL}'
        ]);

Sending direct Guzzle requests with the access_token as bearer in the Authorization header returns the same error:

$client = new \GuzzleHttp\Client();
$res = $client->get('https://www.googleapis.com/blogger/v3/users/self/blogs', [
    'headers' => [
        'Authorization' => 'Bearer {ACCESS_TOKEN}'
    ]
]);

/// returns 403
echo $res->getBody();

I'm really lost. There are no notifications in my Google Cloud Console dashboard. It seems that the Blogger API just broke without a warning.

Anybody else have the same experience or can offer help? Thanks!