Static HTML page next to a WP site

Hi

I have a client who has a temporary static HTML website online.
We’re working on a WP powered webshop at the moment, and that site
will substitute the current one.

The problem is – WP doesn’t display anything correct as long as the
index.html is active.

If i navigate to /index.php i just get redirected to index.html.
WP works if i rename or remove index.html file.

I can’t put the site in a subfolder cause i don’t want to use redirects later on. WP needs to be installed in the root.

Any suggestions are welcomed.