Editor switching automaticalls to html mode

I suddenly (meaning I did not install or change anything) the post / page editor's default mode is "html" instead of "visual" (what it used to be). I cannot find a way to get back to "visual" as the default. I have to select "visual" every time I edit or save anything. At least it works, but it is a pain...
Thanks for ideas why this behavior might have changed and how I can get it back to "normal".

PS: I tried different users without any effect.