Email address not registered. Please register new account first.

[Dibucasiano]

Install the plugin, add the user within the system, when I go to use the google login everything is fine, but when it has redirect me I get "Email address not registered. Please register new account first."

[TBG-FR]

From the README.md :

Add Google oauth 2.0 support to login to MantisBT.
User must have an existing MantisBT account.
Signup / account creation scenario is not supported.

So it looks like you need to create an account on Mantis before being able to login with OAuth... (I know, not what we usually expect from an OAuth element)

[duncanbarnes]

Worth checking your logs, for me this was resolved by fixing #20 (PR coming to fix most of these tickets)