I have the exact same issue on multiple servers and instances of JoomSEF installations. it has to do with it SEF'ing the admin URLs. it also affects module editing.

I reported it here:
www.artio.net/support-forums/joomsef/com...-html5-w3-validation
but no one ever replied.



My solution for anyone interested is to go to plugins, filter by artio, disable them all, then edit, then re-enable. this is very slow and time consuming and end user site clients don't love it either.


I understand this is "community" support -- but it would seem like answering on obviously buggy issues would be good for all.

I also do this step. But this is very frustrating steps.