The problem is that Google's OAuth2 scopes look like URLs. As you can see in the URL, the scope for Gmail is https://mail.google.com/. This does indeed get blocked by a certain option in the .htaccess Maker.
That said, it might also be the case that your host blocks the same kind of URL because of a security rule applied at the web server level. Therefore, instead of telling you to change Admin Tools options I will tell you how to take Admin Tools out of the equation.
Temporarily replace your .htaccess file's contents with those from Joomla's default htaccess.txt. Then, go through with the authentication. After finishing the authentication go back to Admin Tools, .htaccess Maker and click on Save & Create .htaccess.
If you replaced the contents of the .htaccess file but still have the problem, it's your host blocking you. Contact your host. Once they address the issue on their end, follow the instructions in the previous paragraph afresh.
Let me clarify that you only need to do this once, when authenticating to Google over OAuth2 for the first time. After that initial authentication, your servers sends the Access or Refresh Token to Google. There is no longer a callback URL to your site to get potentially blocked. The callback URL is only used in the initial authentication. Of course, if at some point you decide to re-authenticate to Gmail you will have to do the same process again.
Finally, I would like to explicitly state that I am very positive that this will work. We have the same kind of Gmail authentication in Akeeba Ticket System (ATS) for years. These are the same instructions I have given to ATS clients over those years. These instructions are battle-tested in the real world :)
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!
