Hi,
Does this happen as a user try’s logging into your captive portal?
If so this is not an error, it is simply your clients web browser telling them not to trust the certificate your server/captive portal has provided as it it not from a trusted issuing authority.
You have several options ask the user to import and trust the certificate, purchase a trusted certificate or disable the https login (not sure if this can be done – very new to this) this would not be a good idea either as it is then pretty easy for attackers to see the username and password.
Hope this helps
Darren