Not that I know diddly squat, but I have a theory that probably will make no sense to the rest of you and you'll think I'm stupid.
This seems to be happening only on cumulatives. It doesn't seem to be happening on other updates. Both last month and this month I have followed the posts and it seems every user I've looked at that has a profile of his specs has had some third party AV.
I had this same problem with last months cumulative update. It started with the cumulative update preview. Tried every way in the world to get it to apply through WU. (SFC, turn of MWB, etc,etc) I waited for it to come out on update Tuesday, it failed again. I then downloaded from MS server but it still failed. I ended up restoring an image made one day prior to the day I originally tried to apply the update preview (when trouble started.) (some people have done a repair install instead to get it to work.)
After image was restored, I disabled MWB, & just to be on the safe side, Defender as well. (I think any other AV should be disabled here). I applied the update I downloaded and boom, it installed first time.
My theory is this. I think if one uses any third party AV, it has to be turned off before applying one of these cumulatives through WU (even though we never had to before) I also think for some reason once one of these cumulatives fail, some sort of flag or bit is getting set that prevents it from installing no matter what anyone does or how many times one tries. Both a repair install or restoring from an image would clear this bit or flag.
Is my theory so far fetched it's in the Twilight Zone? Am I stupid for thinking it may be happening?
