When a chest/container is replaced via WorldEdit, the server prints out a messages in the console which seem to have no purpose. After talking to WorldEdit devs, they stated this would not be a WorldEdit error, which can also be read here: https://github.com/EngineHub/WorldEdit/issues/1312
No spam should be printed in the console.
How to recreate
Select a chest with WorldEdit and use //set stone
The message doesn't occur on paper 1.16.5, so they seem to have introduced already a fix which probably might be worth to have in Spigot directly instead.