Referencing this issue:
PaperMC/Paper#1440

Can you please have a look here as a few people are leaning toward LuckPerms being the culprit.
All information in the ticket above.

Nothing there suggesting an LP issue as far as I can tell.

For what it's worth, some of the servers I've been running LP on have been using latest 1.12 Paper builds for a number of months - never ran into this issue. This is also the only report I've seen for this error, which leads me to believe it may be either a custom / not-so popular plugin which causes the CME to fire.

I'll happily look into the issue further if the cause can be identified.