I resolved the problem by tweaking with the pdi file on the NAS for Twonky media server. Was not a DLNA problem in the TV after all.
I have a NAS (Buffalo LinkStation Live) with Twonky Media Server (5.1.2). When I connect my LN46B650 to the NAS, it plays video files for a few seconds and then I get this message 'Media play is disconnecting device Twonky LS Pro'.
Does any one have any fixes for this?