Tvheadend

Presumably, the files play okay if you play them over an NFS mount or move them locally? I'm no expert, but it looks to me like this is a communication error between the addon and the backend, but I guess you'd worked that out yourself. What I don't know is whether this is due to congestion on your LAN, some hiccough on the OE box or the backend, an I/O bottleneck (is your backend doing anything else when this happens?), etc. Could something be saturating your LAN connection on your server, are you running a backup, streaming HD files, or otherwise hammering the box, the hub it's connected to, the LAN segment...?

There was also a comment on the xbmc forums about a similar error message, and this was related to clock difference between the front end and back end (I think, the forum is down at the moment so I can't check).

Not necessarily much help to you, though...

Good points, though I fear I can rule out an overloaded network/server for the playback period. The file in question might be a problem, though it plays fine e.g. in VLC or mplayer. I'll have to try and play it through XBMC directly to rule out a player issue. I'll also check the clocks, you never know. Plus run debug logs front and back.

One more observation - the error occurs when resuming the front-end from sleep, maybe the hts plugin and the backend are out of sync on some information, especially if recordings have occured in the meantime. I also notice the "new recordings don't show up in their respective folders, but are listed with the folders on the recordings root directory" issue after resume, feeding my "out of sync" suspicion.

Disable and re-enable the addon, perhaps? It's another test, and one that affects a very specific component...