Files Stopped Playing

Hi – I am using Kodi 17.1 Krypton on an Nvidia Shield. There are some files (avi’s) which are episodes of a TV show, all in one directory, ‘Season 7’. These files, which I ripped from a DVD purchased from Amazon, have always played perfectly well, first on a WDTV Live box and then on the Shield (they are held on a Synology DiskStation NAS, all connected via ethernet through a Gigabit switch). Suddenly these files – and, it seems, only these files – won’t play. Out of interest I installed SPMC and the files play on that without any trouble. I must admit to having one problem after another with Kodi on the Shield, and in desperation I tried using Plex, but that was more or less completely useless. The common factors here are, obviously, the NAS box and the Shield. However, I have the had the NAS for a while now and as I mentioned, it worked with the WDTV box with no problems (and without Plex’s constant irritating complaints about ‘transcoding’).
Any ideas?