When Jenkins is configured with a proxy (and more importantly a "No Proxy Host" list) the plugin cannot match the Bitbucket server address against the noProxyHost variable. This means that the proxy is NOT ignored and when the plugin tries to connect to Bitbucket it fails.

After further analysis it turns out that the

private void setClientProxyParams(String host, HttpClientBuilder builder)

method is not stripping the "scheme://" from the host parameter. This cause the no proxy lookup to fail. What needs to happen is, the scheme must be stripped before comparing it against the lookup list. The host parameter comes from the 

private CloseableHttpClient getHttpClient(String host)

method which requires the scheme:// format.