I also have this issue. It happens in a group when there are more files than _Number of media per page_. E.g. if number of media per page is 10 and there are 20 files in a buddypress group library, there is a fatal php error.
The reason seems to be with the rtmedia_wpseo_og_desc hook. It expects an array, but once there is +10 files, it gets an object instead.
RTMediaInteraction.php Ln: 307: $rtmedia_query->media[0]