We've run into an issue with the plugin as it appears it expects the server URL defined in the Bitbucket endpoint configuration to be the same as the one to be used for cloning purposes even for SSH.
 
So for instance if you have defined https://bitbucketserver.example.com/bitbucket as the server URL, when cloning via SSH it attempts to use ssh://bitbucketserver.example.com:7999(where 7999 is correctly read out of the clone link URLs returned by the Bitbucket API). In our case however, the web URL is different to the SSH url. For instance, based on the previous example, our SSH address is ssh://actualservername.example.com:7999/
 
Given that Bitbucket (Server at least) returns the clone URIs in the Repository API response which is retrieved anywhere, it might be best to use the returned URIs for cloning rather than try and "guess" the correct endpoint.