Change Validation Error Message when Logged in as Administrator and make it more specific

AFAICT, the (nearly) only case where it happens is, when I submit a form that was built in another session (i.e. was long open in my browser, while I logged in/out in another tab to switch admin/normal user or the like). I wonder whether there's a compact-enough way to say "most probably you've logged in/out after the page was initially loaded, so try re-loading a fresh form, and fill it again"

This is because the form CSRF token expired. The message assumes an administrator is able to figure that out, because it assumes the administrator is a developer. That may be a wrong assumption, but saying "the CSRF token expired" would probably be similarly bad: it'd scare administrators who aren't developers.