The RR account was set up by Marten, with me as (co-)adminstrator and Peter and Johan with their own accounts. It was one installation of WP with multiple users. A few months ago, I took over the whole RR account from Marten and the others closed their accounts. The theme was set to Thesis. But this all happened a few months ago (december). I had about 20 new posts since then. Since the last posting and this problem no changes have been made. I am very sure it worked until Friday night (Dutch time) because I visited my own site then.
I can access the site's control panel and already checked it. Usually if there is a problem, I can see it in the error.log, but there was nothing now.