Gallery fails to thumbnail .mkv file and media player won't play it

[John Dolz]

I'm having trouble playing a rather large file (954.6 MB). Upon selecting the gallery for playing, it freezes and creates a qt_temp file and that eventually grows to the same size as the original file. After that the video appears in the gallery, but no thumbnail is generated, and when trying to play it, the media player app opens, showing a black screen and the play controls at 00:00 TC. No video plays.

If I try to load the file via the Terminal (ubuntu-app-launch mediaplayer-app [path-to-video]) the player immediately complains with a "Fail to connect to video backend" error.

Release: Ubuntu 15.04
OTA 8.5 (BQ stable)

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This is the relevant metadata, as reported by SMPlayer:
Container: MKV (Matroska demuxer)

Video codec: ffh264 (FFmpeg h.264)
Resolution: 1920 x 1080
Aspect ratio: 1.7778
Format: avc1
Bitrate: 0 kbps
FPS: 23.976

Audio codec: ffaac (FFmpeg MPEG-2/MPEG-4 Audio)
Format: MP4A
Bitrate: 128 kbps
Rate: 48000 Hz
Channels: 2

It contains 1 audio stream and 1 subtitle track of type SUB.

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The file in question is the HorribleSubs version of episode 6 of the show One Punch Man, 1080p file, available through their site. Please consider the licensing and legal situation of this show in your country before downloading.