I tested the demosite too, and it was fine. I Still use 1.8.2
Yes, I use a custom template, but dropped in the professional again to test.
Still the same problem... And as far as I can remember the admin templates is untouched, only site-wide css is changed.
But discovered something strange:
If I rightclicked the link "contribute" and opened it in a new tab, and waited until the tab was fully loaded before switch, both texareas had the editor.
This was the same for both stock and custom template.
This is beyond my competence, will be waiting for a genie to pass by...