So I'm running into this issue right now. After the authorization pop-up process, the window stays blank with a squarespace logo on the bottom.
After looking at the source code for the window, it actually looks like there's a connection error: "{"error":"Could not find the requested ExternalAuthenticationRequest with name: <lots of jumbled letters and numbers>","notFound":true}"
I don't think this was an issue a year ago. hopefully someone saves us!!