I had a similar issue some time back were apache did not picked up new changes caused by a hanging apache process. Please try to stop apache and then check with:
ps -aux | grep apache
ps -aux | grep http
that no apache processes are running anymore. If you still find apache processes, then kill them or restart the server.
If thats ok, then check if the symlink in /etc/apache2/sites-enabled/ points to the correct vhost file in /etc/apache2/sites-available/
A third thing is the dns, check that the dns record for files.example.com really points to the correct IP address. And dont mix * and IP addresses in the website settings in ispconfig on the same server. If you mix them, all * records will get ignored.