I was setting up Plastic SCM for the first time today. I first installed the Team Edition client, with the "server components" option unticked as my teammate was planning on hosting a local server. While he was setting that up, I decided to try the Cloud Edition client. At this point, I hadn't uninstalled my previous Team Edition install and launched the Cloud Edition installer, which prompted me to replace the existing installation - I confirmed.
When I launched Cloud Edition and attempted to set up a new repo, I received the "No connection could be made because the target machine actively refused it 127.0.0.1:8084" error. After doing an uninstall and reinstall, everything worked fine. I noticed that in the old install, there was no "server" folder in the PlasticSCM installation directory, and I believe this may have been due to my previous install which unticked "server components". Hopefully this information can be of use to the Plastic devs, as this sounds like a very specific case bug that could confound and turn off a new user.