Hi,
We do not know what the exact cause is, but I have had this issue after installing codec packs or vlc. If I use OpenLP straight away I found I got the error, but after a restart it went away.
So if you try uninstalling and installing any codec packs or vlc, with restarts inbetween you may find that the issue resolves its self.
Note: OpenLP 1.9.10 is now out