Can only use commands when upgrading to 1.16.3

[PapillonUK]

My friends and I have spent weeks creating a minecraft server with armour stands as the main focus.
It has been on 1.15.2 for about half a year now and I upgraded to 1.16.3 today. We can no longer edit armour stands using the flint, and only commands work. Whenever we try to select anything using the flint, this pops up:

We are all pretty devastated as weeks of work have gone into this server, and now we can barely edit the armour stands.
Is there a fix to this that isn't downgrading versions?

[RoboMWM]

and this happens on a fresh server with no other plugins? Also providing /version and /version armorstandeditor output would help