I was seeing this behavior too just today. My details are below for reference. I found the issue to be that the AP system profile had the Corporate DNS Domain name set to the internal domain. When I removed this, the captive portal came up fine.
- New setup with a Guest SSID in split-tunnel mode on a RAP
- When setting the SSID in split-tunnel mode, i get the same error as original post
- When setting the SSID in tunnel mode, all works fine
- Confirmed that settting the Captive Portal to HTTP works fine
- Code is 6.1.3.1
Chris