I think a CORS configuration is missing on Gladys Plus, since the backend is at a different URL.
Firefox is refusing the connection because of this error.
Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://api.gladysgateway.com/users/login-salt. (Reason: CORS request did not succeed). Status code: (null).
The problem is that the UI shows an incorrect error message, which is misleading.