The problem is related to a memory bug in QMidi which occurs when exporting using very large video size.
Next version will correct this. In the meanwhile, the workaround is using lower video resolution, maybe 1280 x 720.
QMidi allows using custom video sizes but it's better to use recommended video resolution and aspect ratio:
Also, use either H-264 od MPEG-4 (options button in the export dialog) for better results and compatibility.