User-agent: Mozilla Thunderbird 1.0.7-1.4.1 (X11/20050929)
The "why" is you've included a plugin that creates an actionSet and
specifies a menu. It looks like the menu is created by an extension in
a plugin you DIDN'T include.
That's why there's no errors reported in the eclipse case, just in your
RCP case (which includes less plugins).
You need to find the plugin that provides the menu and include it to get
rid of the errors ... but they're probably describing functionality that
won't work propertly in your RCP app, but that you didn't plan to use
anyway.