Mikhail, it's probably because MEP2015 doesn't have the decoders for the HEVC/H264 MPEG-4 codec, which iphones record-in unless changed in the camera settings to AVC/H264.
Could you give us the MediaInfo readout of the problematic MOV file. Instructions for using MediaInfo are here.
The solution will probably be to convert the file to AVC/H264 using something like Handbrake.
For future videos, I suggest you change the iphone Camera setting to record in "Most compatible" as opposed to "High Efficiency". "Most Compatible" should be openable in MEP.