Unknown SSL protocol error

I want to use payplug paiement plugin on my website. But the config page show me this error:
CURL error (35) : Unknown SSL protocol error in connection to https://www.payplug.fr:443

And in the public page (https://intranet.oveo.org/payer-votre-adhesion/), the result of the shortcod is :
Impossible de générer le shortcode PayPlug, merci de vérifier la configuration PayPlug

ANy idea ?
thanks,
Julien

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