Hugo5000

When executing with the console one can see it clearer because it is printed directly between command and summoned message

summon minecraft:text_display 1 1 1 {text:'{"text":"test"}'}
[18:26:57] [Server thread/ERROR]: Display entityNot a string
[18:26:57] [Server thread/INFO]: Summoned new Text Display

summon minecraft:text_display 1 1 1 {text:'{"text":"test"}'}
[18:26:57] [Server thread/ERROR]: Display entityNot a string
[18:26:57] [Server thread/INFO]: Summoned new Text Display

It is caused by the command and can be replicated easily, it was just that my server was lagging; which is unrelated to the message

And as far as I know it is only the text that is required, the others have a default value

I'll just do it again and execute the command a few more times so one cant say "its not related to the command"