I had to deal with an issue with a wireless network guest portal for a customer recently that had me and TAC stumped for a month. The splash page seemed to load fully, but there was always a small spinning circle in the center suggesting something on the page didn’t quite finish loading. It resulted in end-users seeing an intermittent but frequent error when they connected to the guest wireless getting the redirect URL to the guest portal page on Cisco Identity Services Engine 2.3. The error was
[ 400 ] Bad Request,The request is invalid due to malformed syntax or invalid data
and here’s what I did to troubleshoot and eventually fix it.