I have a forum on a hosted instance and I am not seeing this error.
I notice it isn’t happening in safe mode on your forum, so it is likely a plugin or component.
I did just create a new category… which led to me discovering the problem. So maybe there’s an odd sideeffect to cat creation . . .
I do have two custom components installed (that I wrote)… disabling those didn’t fix the problem. But works-in-safe-mode is a start at troubleshooting.
Just closing the loop here - @merefield has now made the necessary updates to the Pavilion theme component, so it should work well both before and after the incoming core change