Yes. Server has been restarted several times with these settings in place

threads pls

My IP address is added to the list.

Is going via yer usual nginx web server (or whatever yer using) vs the inbuild webserver any different?

Using IIS.

(I am clutching at straws here: not seen anything like this, but also have not used CF in prod since CF9 so my knowledge is creaky)

Do you by any chance have

this.enablerobustexception = false

in your Application.cfc?

this.enablerobustexception = false was it! Thanks for the insight

^^^ TIL