Hmm. Does it fail like that every time?
It is a failure while fetching data - that is, before applying any changes - so your system is in no danger.
I would thing that the update command has verbose flags, that would tell you what file failed, at least. Hmm, checked, and it doesn't! Blind Alley there.
No, I just had to read the documentation - that is, the file itself.
-v is for debug levels. Your options are debug, stats, and nostats.
So, try it again with -v debug, and let us know what happens. (You will get screen after screen of nonsense: Maybe use
script(1) to save the output for later perusal.