Piwik only returns index.css (no replies)

Hi everyone,

I installed Piwik on an IIS8 via the Web Platform Installer (http://piwik.org/blog/2010/08/piwik-on-web-app-gallery/). After the installation I try to open myapplication.com/piwik and I see the first page of the Piwik installtion process. However when I click "Next" I only receive the Index.css File. In IE I am asked if I want to download it, in Chrome I see the contents of the file:

/* compile_me_once=76f5519695dde365532c3ee4eeef35bd */
/* Piwik CSS file is compiled with Less. You may be interested in writing a custom Theme for Piwik! */
.clearfix {
*zoom: 1;
}
.clearfix:after {
content: "";
display: table;
clear: both;
}
.clearfix:before {
content: "";
display: table;
}
[...]

Any ideas what might cause this behavior?