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You need to make sure that:

1. you able to rich out your sitecore instance by http protocol
2. site https binding "Require Server Name Indication" setting is unchecked
3. you don't have any URL Rewrite rules for redirect from http to https

After all these steps you can create a new connection:

Just checked it on my local env on the same Sitecore 9.1.0 instance and it works fine.