[TYPO3-english] femanager - Captcha field not used checked?

[JZ]

Using femanager (version 1.5.2 on Typo3 6.2.19), I added the captcha field in Flexform and the following in TS:

plugin.tx_femanager.settings.new.validation.captcha.captcha = 1

sr_freecap (2.3.1) is installed and the captcha is effectively displayed on the registration page, the * is also visible beside the title.

Unfortunately when I submit the registration form, captcha is not checked at all. I can let the field empty or enter what I want, the registration is done!

Am I doing something wrong here? Or is that a known problem?