Did you ever get a reply to this or get the problem sorted?
Seems to happen to me with anything which is over 1gb and saved to memory card. I have tired copying the same videos to the internal memory and this doesnt solve the problem and have tried a restart. Have also tried different apps other than the standar movie player on the phone. It doesnt show the mp4's as videos which can be played though they are visable on file commander. All my software is up to date an have searched google for a solution. There are other videos I have which play off the memory card. is there even an sony app/program that will convert it if it is the codecs that the video file use. I dont have a problem playing the files on any other devices.