backup validation, corruption

I can't tell you what the problem is but it is likely that the backup image is fine and not corrupt in the sense that you will not be able to restore using it.

I had the same sort of issue (failed validation) but restoring from it worked just fine. It was suggested to me that maybe one or more of the non-critical files was corrupt but not essential system files.

OK, that may be the case, but the uncertainty isn't too appealing...I mean, suppose the file actually is corrupt in a serious way?

Yep, agreed - not very encouraging at all.

And I was also getting a message at the end of the recovery process to the effect that "recover operation failed". But I pushed on anyway, rebooted and all was well.

I don't get the failed validation anymore (but don't know/understand what is different) and I still get the recover failed message. But it does work nonetheless. And just to be sure, I have wiped my SDD by over-writing it, rebooted and nothing happens (except error messages of course), used the disk image to recover from, got the recover error message, but upon rebooting, all is well and everything is indeed recovered - OS, customizations, programs, data.

So yes, not encouraging, but it still seems to work. Not sure I'd recommend this sort of drastic testing (overwriting the HDD) to everyone though!

No, I don't think I fancy that level of testing myself :-). I did look at the famous Acronis flowchart for corrupt files, but it was not very helpful. I am wondering whether I should roll back to 2012 which never gave me any probs at all (and from which I successfully recovered my system without a hitch).

The problem with rolling back is that older editions of TIH will likely not support the newest versions of Windows.