HTTPS social network redirect captive portal

Hello,

We have a FortiAP 221E managed by Forticloud and we have configured our external captive portal.

We have login by e-mail and social networks like facebook, instagram, etc.

E-mail login is working good, but when user tries to connect via social network, the captive portal shows a HTTPS certificate error.

Have you idea of what could it be? I'm i missing some configuration in Forticloud or does the AP needs some kind of configuration too?

Thanks!