First, thank you so much for the quality of XnView, I'm a long-term fan.
Then, I've often seen XnView crash if there's some .mkv files in a folder (not all)
Maybe related, maybe not: one of my folders contained an invalid .mp4 file (32 bytes long): it crashed.
You can easily reproduce this by downloading and unzipping this: http://cyph.info/temp/Untitled.zip (150 bytes)
Y-Phil
