By following the modsecurity log it appears that this file was breaking two mod-security rules…disabled them for this site and the editor is fixed…thanks for your help

There is no “hosting provider” per se. Site is hosted on a virtual server that we fully manage. We have root.

Anything in particular that we should be looking for ?

I should add to this, that it is possibly some sort of conflict with block editor. When I tried it from a different browser as per:

I got a pop-up that said something about “welcome to the block editor”. Maybe that is it?