Wim,
JIC case this got lost in the shuffle. There is an issue with the Menu/ribbon in the header not showing on certain mobile devices. I have verified that the ribbon disappears or re-appears depending on the width of the browser window, and that rotating a mobile device to Landscape will show the Menu/ribbon. mldenison had same issue.
Ribbon disappears when the browser width is <640px, @media screen and (min-width: 640px) - W34_index2.php. Something must have changed here since April version.