I'm working on a cloud based email system (Amazon Linux instance, written in PHP) that connects to Gmail using using OAuth2 and then simply pulls messages. For most users, this has worked fine - we were able return an access code, generate access tokens, and fetch messages. But now, when users try to grant our application access via the web interface, the server returns an error message, and no access code:
"Warning: Google prevented a suspicious attempt to sign into your account. Was it you?"
What is going on here? Why have we been able to connect in the past, and now we are being blocked by Google? I do not have any more information to clarify the problem. Explanation of some standard cases in which this might happen would also be helpful.