nginx cache - could not allocate node in cache keys zone

[CJ Ess]

I have four servers in a pool running nginx with proxy_cache. One of the
nodes started spewing "could not allocate node in cache keys zone" errors
for every request (which gave 500 status). I did a restart and it started
working again.

What conditions cause that error in general?

If it happens again is there anything I can do to try and determine the
root cause?
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