Welcome aboard to the forum
I have heard about users having problems playing back Princo media in certain players, however so far I haven't across any mention of whether these play in LG models. For a start, I would recommend giving a different brand of media a try, such as Verbatim, Datawrite, TDK, etc. which seem work well with most standalone DVD players and recorders.
Generally an unknown media message means that the player has detected that a disc is loaded, however it is unable to begin reading what's on the disc. Chances are that you would get a different error message if it was due to the disc's content, such as an invalid disc message or some other error indicating that the disc has no playable content. Some DVD players such as certain Lite-On models will actually start playing any VOB files present on the disc if no header files are present.