Grilo is a library for accessing media files from various sources (including UPnP) that is used by some gnome tools (rhythmbox, gnome-music and I guess some more) and for example also Ubuntu Touch. Probably not all of them use the UPnP feature of grilo.
Under certain conditions (when a filter is set), grilo-upnp uses the "Search" action with @parentID conditions for browsing instead of "Browse".
It seems like ReadyMedia does not support @parentID in a search query, though that feature could be easy to add, since @id and @refID appear to be supported already and it seems to be supported by the SQL schema. So it might be a few lines patch, but I don't know the source well enough to be sure.
Adding @parentID support for searches should improve compatibility with any software using grilo.