After upgrading Apple computers to macOS "High Sierra", the system disconnects if a captive portal system responds with HTTPS.
To work around this issue, configure the NAS (controllers and/or switches) to redirect using HTTP instead of HTTPS.
Secondly, if using ClearPass Guest as captive portal, disbable "Require HTTPS for guest access" in ClearPass Guest under CPG -> Configuration -> Authentication.
Note that the NAS login can still use HTTPS, so that credetails are posted securely into the NAS.
Apple is aware of the issue.