tscwechselschritt
I guess you're using editor now when editing that custom HTML module, the default editor often strips out the HTML tags.
You can go to Global configuration >> and disable the editor first then edit module again
Here is default content of this module:
<ul class="social-list">
<li><a href="#" title="facebook"><i class="fa fa-facebook"></i></a></li>
<li><a href="#" title="instagram"><i class="fa fa-instagram"></i></a></li>
<li><a href="#" title="twitter"><i class="fa fa-twitter"></i></a></li>
</ul>