Hi Stephan, I sometimes found that headings with the mw-headline class caused links above it to become non-clickable. It may have been due to an attempt to prevent anchor links from being buried beneath the upper navigation menu, creating a margin of 50px between the top of the page and the anchor link (it's quite possible that the issue occurs only when the font you're using for subheadings has a generous margin of its own). Anyway, whatever the exact cause of the problem, there's an easy CSS fix for that I think, which is to substitute "block" for "inline-block" - unless it creates other issues I'm not aware of.
.layout-fixedhead h1 > span.mw-headline::before, .layout-stickyhead h1 > span.mw-headline::before, .layout-clean h1 > span.mw-headline::before,
.layout-fixedhead h2 > span.mw-headline::before, .layout-stickyhead h2 > span.mw-headline::before, .layout-clean h2 > span.mw-headline::before,
.layout-fixedhead h3 > span.mw-headline::before, .layout-stickyhead h3 > span.mw-headline::before, .layout-clean h3 > span.mw-headline::before,
.layout-fixedhead h4 > span.mw-headline::before, .layout-stickyhead h4 > span.mw-headline::before, .layout-clean h4 > span.mw-headline::before,
.layout-fixedhead h5 > span.mw-headline::before, .layout-stickyhead h5 > span.mw-headline::before, .layout-clean h5 > span.mw-headline::before,
.layout-fixedhead h6 > span.mw-headline::before, .layout-stickyhead h6 > span.mw-headline::before, .layout-clean h6 > span.mw-headline::before {
content: "";
display: inline-block;
height: 50px;
margin-top: -50px;
}