I think memoize just rolls a die to see whether to abort or not. I can't reproduce it reliably. I can't make a minimal example. I can't even identify a particular extern, though some seem more susceptible than others. Even when it happens, the number aborted seems arbitrary. Sometimes 12. Sometimes 3. And when I manage to catch it, the tracing info looks identical except that one aborts and the other completes. The closest message seems to be

 [tracing advice]   Executing outer handler: \mmz@pgfsys@getposition

but there's another message from chronos before it reports completing/aborting.

@SašoŽivanović Is it expected that \mmzset{path={dir=memos}} is no longer recognised?

@cfr Yes, please use \mmzset{prefix=memos/}. No need for a mkdir either.

@cfr This is the culprit, for sure. The ultimate source of abortion is some remember picture, but specifically, memoization is in your case aborted when encountering \pgfsys@getposition, which retrieves the position of the picture on the page saved in the .aux (which contains a \pgfsyspdfmark).

If I remember correctly, I have introduced this hack of aborting upon \pgfsys@getposition to solve the problem of "remembering the wrong picture": in v1.0.0, once a picture was memoized, \pgfpictureid of subsequent pictures was changed, leading to problems. However, in response to Qrrbrbirlbel's trouble, the new version of Memoize (the one on GitHub) increases \pgfpictureid when utilizing a TikZ-extern, so — I realize now — the \pgfsys@getposition might have become obsolete.

Bottomline, can you try deactivate csname=pgfsys@getposition? I'm guessing when this macro is deactivated, things should work in the new version, while weird stuff should happen to the subsequent pictures in the old version (if they use remember picture).

@cfr This is to be expected. Memoize only reports abortion at the end of memoization.

Btw, I think you have convinced me to change the abortion UI to require \mmzAbort{<explanation>}.

@SašoŽivanović With the old version I get

! Package advice (/mmz) Error: Command "\pgfsys@getposition" is already activat
ed.

Does it need to be in the cfg file?

@cfr The latter, i.e. the .aux contains \pgfsyspdfmark with a wrong \pgfpictureid. But if there is no mismatch, \pgfsys@getposition doesn't get executed and memoization isn't aborted. (As I said, these mismatches should be gone in the new version, but I still have to test whether that's true myself.)

@cfr deactivate it.

@cfr No.