With SSL enabled (via a CA-signed certificate) and the “Force Secure Redirect” option checked, I took a gateway backup and restored it on another gateway, which was missing the certificate. Ignition correctly identified that SSL was no longer configured and grayed out the HTTPS Settings pane. However, despite being disabled, the “Force Secure Redirect” checkbox was still checked, and Ignition would not allow me to connect via the Designer over the HTTP port.
The workaround was to enable the checkbox using Chrome DevTools, uncheck it, and click the Save button. A proposed fix would be to ignore the “Force Secure Redirect” setting if SSL is not configured.
Bug confirmed in 8.1.5 running on Linux.