1) Like with pre-Plex 0.9.5 and Plex 0.9.5+, where Plex Inc. changed the bundle identifier of Plex for the first time, Remote Buddy will only address the newer one. I've explained the reasoning for this before in this forum. If you want to use an older version of Plex, please first remove the version you don't use, relaunch Remote Buddy (so it no longer finds the newer Plex/HT on startup) and you can control your old version of Plex again.

2) Plex/HT also uses a different communications port than previous versions of Plex. The updated Behaviour takes care of this. You can, however, manually (via a text editor) configure Plex/HT to use a different port or use firewall software to block access to that port (remember: Plex/HT uses a new bundle identifier (actually two depending on the version of your copy), so your old firewall settings for Plex will not apply to it! You'll need to set these up anew.).

I'm working on having the Behaviour detect even the rare case of a manually configured, different port and handle it. What I'll not be able to detect or work around though is firewall software blocking control packets (since these are UDP, the Behaviour doesn't get any information on whether the package was delivered).

Please check your system's OS X firewall settings: does Plex/HT have permission to accept incoming connections? 
Do you use any third party firewall software like Little Snitch? 
Please also run this command in Terminal.app and copy and paste its output:

sudo lsof | grep UDP

Thanks in advance.

Best regards, 
Felix Schwarz