Custom login page does not redirect to wp-admin/ 404

Hello,

I have searched for this issue but it does not seem to be reported yet.

We have WP Cerber installed on various websites configured with the custom login page, and with wp-login.php blocked.

The issue did not arise recently but exists since various years at least.

Sometimes (not every time), and especially on first login (logging out and logging in again works well) the following happens:

1. Access the custom login page
2. Fill in the form, submit.
3. A 404 error appears and the URL has not changed.

Normal behavior is the following:

1. Access the custom login page
2. Fill in the form, submit.
3. No 404
4. Redirects to wp-admin/ on successful login.

Any idea of why?