Goodafternoon rdekruijf ??
Ok, I now fully understand what is happening.
On second thought I don’t think the captcha is an active plugin in the WordPress env.
It looks like this is either part of the Softaculous install or an extra feature for WordPress sites which is included in the hosting providers package/service (which basically boils down to same thing).
You can probably find out exactly what it is by clicking on the link underneath the captcha form:
Informatie over deze beveiliging
Perhaps logging into the account management system also allows you to disable/enable or tweak the captcha feature.
In any case entering the captcha correctly once initially returns a 404. But a subsequent second attempt to access the same secret login url will provide immediate access to the WordPress Dashboard login screen (the captcha form is skipped).
So I’m pretty sure the action behind the captcha form tries to redirect to the wp-login.php script. Since the iTSec plugin Hide Backend feature also blocks (actually responds with a 404 to) wp-login.php requests a 404 is returned upon a correct captcha submit.
Basically the captcha form is unaware of the iTSec plugin Hide Backend feature responding to wp-login.php requests with a 404.
If you could somehow tweak the captcha form to redirect to the secret login slug as set by the iTSec plugin Hide Backend feature the problem would be solved.
Another option would be to disable the captcha form.
dwinden