I have tested this on several UBB powered boards, for which I found the links on this forum. And I noticed that they all have the same problem, except for this support forum.

If I register as a new member and make a mistake with the captcha, I get a warning and are returned to the registration screen to retype the captcha.
From that moment on, it's no go.
You can type as many correct captchas as you like, but it will keep telling you that the captcha is wrong.

The only exception is this support forum.
After correcting (retyping) the captcha and sending the form, I get a blank screen. That's it.
The address bar reads:
https://www.ubbcentral.com/forums/ubbthreads.php
But nothing further happens.