Gmail SMTP Error

Hello,

when trying to send testmail I am getting error code 400. I already looked up the issue and tried everything to limit possible reasons, because I don’t it’s a wrong or missing client ID.
Here the error:

Versions:
WordPress: 4.9.4
WordPress MS: No
PHP: 7.2.2
WP Mail SMTP: 1.2.5

Params:
Mailer: gmail
Constants: No
Client ID/Secret: Yes
Auth Code: Yes
Access Token: Yes

Server:
OpenSSL: Yes
PHP.allow_url_fopen: No
PHP.stream_socket_client(): Yes
PHP.fsockopen(): Yes
PHP.curl_version(): Yes

Debug:
Error while sending via Gmail mailer: {
"error": {
"errors": [
{
"domain": "global",
"reason": "failedPrecondition",
"message": "Bad Request"
}
],
"code": 400,
"message": "Bad Request"
}
}

Configuration has been done by simply following your instructions. No other SMTP or caching software is installed. Everything is up-to-date. Somehow I am assuming it might have sth to do with security restrictions I added. See error code line: PHP.allow_url_fopen: No
I already went through my .htaccess and functions.php to look for security restrictions I set. Neither it was the implemented 6g firewall or xss-header configuration, nor the PHP.allow_url_fopen setting. I changed to ‘Yes’, but with same result.
If you have other ideas leading to the goal, they are highly appreciated. ??

Thanks for your help,

-Bj?rn

The page I need help with: [log in to see the link]