Possible apt update blockage from use of aptcache?

[timrowledge]

I'm having a very annoying time trying to update the set of Pi that I configured with sdm and the aptcache plugin. Initially it seemed perfectly fine, then a few months ago(?) there was a short period where any attempt at updating would fail with complaints about the repository/password/sometihng. It went away on its own shortly after, so I forgot about it.

Now the problem is back and being very annoying. I've whacked around with google to try to find suggestions but none of them make any difference :-( It's clearly sometihng that occurs for a lot of people running a lot of variants of linux, so not a Pi-specific thing.

However, my latest test was to use the same base .img that I had done my sdm work with, write that to an SD card, stick it in a Pi 5 and (after basic manual setup) do an update; no problem. The obvious difference relating to updates is that this version has no connection to the aptcache thingy.

So my first question is how to turn off the aptcache client? If I can do that (preferably non-destructively) on one of the affected Pi and it solves my problem then we have learnt something potentially useful. I have the same issue on the Pi running the aptcache server too, which may indicate that I'm chewing the wrong bubblegum.

There is a short thread about this on https://forums.raspberrypi.com/viewtopic.php?t=377277 which may be useful. Help me Obi-Wan, you're my only hope!