Hi Pavel,
many thanks for trying to help. I should have mentioned, that I'm using a 3rd party template, which is based on Helix Ultimate, but obviously uses different styling options. But your input gave me the right hints where to look at and I could figure it out for myself. Here is the solution, in case somebody searches here for the same issue:
@media (max-width: 767px) {
.uk-hidden\@m { display: block !important; }
.uk-visible\@m { display: none !important; }
}
@media (min-width: 768px) {
.uk-hidden\@m { display: none !important; }
.uk-visible\@m { display: block !important; }
}