For everyone who has a similar problem, this is not a certificate at all. The server may close the connection or return an error code.
For more information, first click file -> clear log in the web connector. Then try adding your application again. After that, click the view log button in the upper right corner. Scroll down for more details on what happened.
You will see something like remote server returned an error (401) unauthorized or a similar message, which is much more useful than what appears in the dialog box.
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