This may or may not be the issue, but in some cases it’s possible to have a designer stuck in this situation if your gateway does not have SSL enabled but Force Secure Redirect is enabled. If your gateway does not have SSL enabled (using an http url instead of https) navigate to the Config > Web Server and see if Force Secure Redirect is enabled. It will look like:
If you are in this state, you can either enable SSL temporarily so you can disable Force Secure Redirect before disabling SSL again or navigate to your gateways data/gateway.xml file and set the entry for gateway.forceSecureRedirect to false (this requires a gateway restart). After that, your designer should be able to connect again. This is a bug currently on our end currently waiting prioritization.