This posting is older than 6 months and can contain outdated information.
The "Navigation: Previous Menu" sends the same button event packet to
Plex as the Apple Remote support that comes bundled with Plex itself
(and which the usage of the Virtual Remote did trigger). "Previous
menu" sends the "previousmenu" XBMC command to Plex. Both are
interpreted and handled by Plex itself.
I see three possibilites that this can go wrong:
1) Plex' keymap.xml file
Did you modify the keymap.xml of Plex (or try to move it away and have
Plex recreate it with its default settings)? The keymap.xml defines
all reactions to button event packets that Plex is receiving.
2) Wrong mapping in Remote Buddy
Also, did you try to reset the mapping of the Plex Behaviour to its
defaults?
3) The Harmony sends other Apple Remote button codes than you expect
for a particular button, consequently also triggering a different
action.
Please try with a stock Apple Remote / Apple Remote Harmony profile.
Best regards,
Felix Schwarz