"Error The page you requested was not found

I am Getting this error “Error The page you requested was not found” on either Saving a Template or a Saving a Post using RTE field (Most of the Time if i copy paste text from Word Doc or Justify Text etc.), it stated when the server was upgraded to Php 8.0. Also NOTE that this error sometimes does not happening and i can save posts

How do i resolve this Issue or steps i should follow to resolve this issue?

Paul