a) I downloaded and installed Plex Home Theater (PlexHomeTheater-1.0.4.144-e40f343d-macosx-x86_64) and I could control it with Remote Buddy 1.24.2 without any issues.

b) I then quit PHT and removed all configuration data, as to ensure I wouldn’t miss anything here. Launching PHT again, I can still control it with Remote Buddy 1.24.2 without any issues.

Therefore, chances are, you’re running into one of many possible scenarios where Remote Buddy won’t be able to control Plex Home Theater. I'll go through the most common ones here:

1) Remote Buddy wasn’t re-launched after installing Plex Home Theater.

Since Plex 0.7/0.9 and Plex/HT use different UDP ports and support different actions, the Plex Behaviour will look at which version of Plex is installed on the computer and use settings for that. Unless you re-launch Remote Buddy after installing Plex Home Theater, the Plex Behaviour may still use the settings targeting Plex 0.7/0.9.

2) Plex Helper (from Plex <=0.7/0.9) blocks access to the Apple Remote - so Plex HT Helper can’t access it.

You can Remote Control Diagnostics (from the Labs section of this website) to check whether Plex Helper has a lock on the Apple IR Receiver. If so, you can use Activity Monitor to quit and Finder to remove the helper of Plex 0.7/0.9.

3) Plex <=0.7/0.9 and Plex HT are installed on the same machine.

As illustrated in 2), this can potentially cause complex issues in the lower layers. You should avoid trying to use an outdated and the latest version of Plex side by side.

4) Firewall settings block incoming traffic for Plex HT.

The Plex Behaviour of Remote Buddy sends UDP network packets to Plex in order to control it. If the OS X firewall blocks incoming traffic, these packets won’t be delivered. Go to System Preferences > Security > Firewall and check that the Firewall is either turned off completely (OS X default) or - if turned on - Plex HT is granted permission to receive incoming traffic

5) "Corrupt" settings carried over from older versions/betas

Plex has taken on a lot of different names, bundle identifiers, settings and versions throughout its history. If you’ve been following its development for a while, chances are you’ll find a colorful mix of folders starting with "Plex“ in ~/Library/Application Support/ where it stores settings (untypical for native Mac apps) and support files.

Moving these folders to another place (or removing them) while the respective Plex application(s) aren’t running should reset them to default settings.

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Please go through this and let me know if your problem was solved by that. If not, please enter

sudo lsof | grep UDP

into Terminal.app (will prompt for your admin password) while Plex Home Theater is running and copy and paste the result or send it to the email address listed on the imprint page.

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As a point of reference, this is what you see in the output of that command while an instance of "Plex Home Theater.app“ is running with default settings (freshly installed):

Plex\x20H 16738 felix 5u IPv4 0xf6ce5177eefc8a1 0t0 UDP *:* 
Plex\x20H 16738 felix 21u IPv4 0xf6ce5177dfc02e9 0t0 UDP *:32415 
Plex\x20H 16738 felix 22u IPv4 0xf6ce5179bdfddf1 0t0 UDP localhost:55535 
Plex\x20H 16738 felix 23u IPv4 0xf6ce517ac7bc3a9 0t0 UDP mycomputername.local.:49399 
Plex\x20H 16738 felix 24u IPv4 0xf6ce51788cb08a1 0t0 UDP *:32412 
Plex\x20H 16738 felix 31u IPv4 0xf6ce5177f3e6961 0t0 UDP *:9778 
PlexHTHel 16743 felix 5u IPv4 0xf6ce5177eefc8a1 0t0 UDP *:* 
PlexHTHel 16743 felix 7u IPv4 0xf6ce5177dfbdeb1 0t0 UDP *:*

Remote Buddy’s Plex Behaviour in particular depends on the fact that Plex Home Theater listens on UDP port 9778, as can be seen in this line

Plex\x20H 16738 felix 31u IPv4 0xf6ce5177f3e6961 0t0 UDP *:9778

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[UPDATED] Current status of this issue:

I can’t fix what isn’t broken. Everything works fine in my own testing - and early feedback indicates this problem is a cause of "corrupt" settings that were carried over from older installs/previous versions.

Best regards, 
Felix Schwarz