This program was not recorded because of an unknown reason

I've had four Comcast DVRs fail with the same problem. Season passes fail to record, with this message in Recording History:

Not Recorded [...] This program was not recorded because of an unknown reason. If you continue to encounter this problem, try restarting the DVR.

Per the advice, usually restarting the DVR does fix the problem for a while. (a day, or a week)

But in 2 of 4 cases the machine became permanently wedged, and never resumed recording. We had those machines swapped, and now we have 2 that are failing intermittently.

Once the DVR is in this state, the problem can be reproduced by pressing the record button in live TV or in the program guide, but in that case recording fails silently.

Sometimes (rarely) a blue box appears saying something about setting up a recording; this message goes away after several minutes.

Comcast support did see an error in the boot log about wall gardening, which suggests a networking problem on their end, but they don't seem to know what to do about it.

Also I've noticed that the problem occurs only after I use Live TV, or is that just my human mind seeking order?

 

This keeps happening. Also On Demand fails intermittently, and a significant fraction if recordings are corrupted.

I've spoken to several support reps at Comcast who have not got two clues they can rub together for warmth.

Since I've seen the same problems on 2 other units, I conclude these issues are systemic. Since it appears Comcast has lost interest in supporting TiVo UI boxes, I must conclude this is a lost cause.