I got this the other day, and again today - twice in a row:
Clicking on the ‘inform the administrator’ link tries to take me to https://abdlstoryforum.info/%1$s which reports a HTTP 400 error.
To replicate this I, erm, just leave my browser open. It does seem to occur a few seconds after I refresh the page, but not immediately the page loads. Always happens on the page shown.