OK, this looks like a genuine bug. The above exception is caused when trying to use an HttpClient configured by the PKIAuthenticationPlugin, in combination with SSL. Tests using SSL set a static HttpClientConfigurer on HttpClientUtil, but this is then replaced by the PKIAuthenticationPlugin, so any clients created after this replacement don't get SSL support. Maybe HttpClientUtil needs to maintain a list of configurers and run through them all, rather than just having a single one?
This only happens on 6.x, as the mechanism has changed in master.