If I uses this code for a button in the revolution slider, the button does not have the right height in Safari or Chrome (Firefox works fine):
<a href="/wordpress/referenzen" class="button dark_btn sm_button">Referenzen</a>

It’s the second slide on http://reprobau.de/wordpress/

What can I do to fix this?

Have you added any extra CSS to your slider/button, you get a line-height:0 from somewhere, check with Firebug, that’s what’s causing it. IF you can’t find it add this to your css, either one of the files, or your Custom Styles (in theme options):

.rev_slider_wrapper .button{
line-height: 1em!important;
}