Users Receive SSL Browser Error When Directed To Network Login

Goto Authentication -> Services -> Scroll down to 'Captive Portal Settings' -> Disable / Uncheck 'Captive Portal Uses HTTPS' -> Apply

This will essentially redirect users to captive Portal on HTTP which may not be recommended.

Ok thanks for that tip. I just had a thought... is it possible to use a custom SSL cert for this that is signed by a known CA, so that HTTPS can still be used without errors?

No - Because with such a possibility, we can stop using HTTPs in the Internet.

If somebody can intercept your HTTPs Connection without letting you know (because you trust this appliance), you could read everything in the internet.