Some days ago it happened here for the first time, without changing the browser or the profile. Also my internet connection via ADSL is far better than Dave's. The bad is that getting the content back via the 'Back' button doesn't work then. One has to copy the content to the clipboard before pressing the 'Post' button.
Moreover, I noticed the following, also since a few days: The Yoomla main page
www.os2world.com/ won't keep me logged in anymore, while the Simple Machines Forum pages still keep me logged in.
After pressing the 'Log in' button on the main page, I get a blank page with a single message: Invalid Token
I suspect the cookie processing doesn't work anymore for the main page: After I deleted the cookies, it started to work, but just one time.
Maybe these two are somehow related.