After logging in I get an extra layer:
This is fine, but now I have a protected post, and after entering password this captcha shows up. After done it the page redirect to a blank page. One needs to re-access the post from the URL bar, then re-enter password again to read it.
So far I only have two security plugins: Jetpack and Wordfence. But even when I have deactivated them, this still persists.
Do you know how to find what cause this?