Thanks, turning of minification of js has fixed it. Thanks for your help.
I had deactivated sticky headers as a temporary workaround for the problem.
I turned them on again, so please check the link again (perhaps after clearing your browser cache).
I just looked at an older version of my site in a virtual machine that was still running WordPress 4.9. With that, the anchor links are working correctly. When jumping to a header, there is an offset that will place the header under the menu so that it is not hidden.
So this offset calculation seems to have been broken by WordPress 5.
After upgrading my old site to WordPress 5, the offset is now also broken.
I’ve solved this particular problem by using Enfold Icons instead of Unicode symbols (which still don’t work, even on a clean test site).