First you need to find the place that sends you to /cgi-sys/defaultwebpage.cgi
Start with the DirectoryIndex section. You may have a leftover directive from a CMS you're no longer using. Then study the rest of the htaccess. I'm assuming you don't have access to the config file itself.
If you can't find anything, see what happens if you temporarily disable the htaccess file. Either delete it (KEEP A COPY!) or rename it to something like "foobar" --no leading dot, make up a silly name-- that a robot is not going to guess within your two minutes of testing. If the redirect goes away, it was in your htaccess somewhere. If not, it's either in the config file or you've got more than one htaccess to keep track of.
You can't simply add a redirect, or you risk going around in circles. Or, at best, getting multiple redirects on every request.