Sign-in warning for in-app browsers
If you opened a MentionDrop link from inside the LinkedIn app, Instagram, X, or another in-app browser, then tapped Sign in with Google, Google blocked you with a 403 error. That is Google's policy against OAuth from embedded webviews, not something we could fix on our side.
Now the sign-in and sign-up pages detect the in-app browser, name the app you are in, and tell you to open the page in Chrome or Safari before signing in. There is also a one-tap button to copy the page link.