I am attempting to set up a BitBucket Cloud smart mirror instance but can not get past the step where you connect to BitBucket Cloud. Every time I've attempted this I end up getting the following error:

I am using nginx as a reverse proxy server and the SSL certificates are installed on the machine. I have also made sure to add the necessary certificates to the Bitbucket keystore. When I initially connect to the server to configure the base URL, Chrome sees the certificate and marks it as valid.