I corrected the problem by myself.
In the style.css of the theme
find
.reply { font-size: 12px; padding: 0 0 10px; position: absolute; right: 0; top: 10px; }
and replace with
.comment-body .reply { font-size: 12px; padding: 0 0 10px; position: absolute; right: 0; top: 10px; }