%origin% is used in xcu files to reference other resources in a package. Most
often xcu files are stored flat in an extension package and in this case it
works as expected. It doesn't work if the configuration package structure is
reflected in the package itself. In this case %origin% gets expanded to the
directory of the xcu file and resources are expected relative to this directory. 
The behaviour is exactly described in the DevGuide but probably not really the
behaviour that was intended. 

("Use a variable %origin% as a placeholder for the exact path where the file
will be copied by the Extension Manager. When unopkg installs the data, it
replaces the path with an URL containing a macro and writes into the
configuration. This URL has to be expanded before it is a valid file URL. This
can be done using the <idl>com.sun.star.util.MacroExpander</idl> service. The
%origin% variable is, for instance, used by the ImageIdentifier property of
add-on menus and toolbar items, which is described in the Configuration section.")

Three ways to solve this problem
- %origin% will be changed to reference the real package root. This can break
existing extensions
- new %pkgorigin% or %pkgroot% variable that gets expanded to the real package root
- adapt the options dialog implementation to handle relative URLs in the
OptionsDialog.xcu. This solution would help for options pages only and is so far
not recommended