This is a security feature to prevent clickjacking. But it is possible to disable it. Put a copy of /usr/local/Repetier-Server/etc/RepetierServer.xml into /var/lib/Repetier-Server/database and change
<!-- To prevent clickjacking we should disallow embedding in iframes -->
<allow-iframe-embedding>false</allow-iframe-embedding>
If you already have a file version there just add that line. Change false to true to allow iframe embedding.