Well, never mind this one. We are going to change our theme.
For anyone having the same problem:
I only temporary fixed it by removing the header on the mobile. I did this as follows:
In wordpress admin under appearance –> editor
I edited the skeleton.css (scroll down on the right hand side for that one).
I changed:
@media only screen and (max-width: 767px) {
into:
@media only screen and (max-width: 767px) {
#header {
display: none;
}
It is not a real fix, but for my it is sufficient while we rework our website in a different theme.
Of course, after any update of the theme your changes will be lost.