At first, try to add Veeam Reporter's web-site into list of Trusted Sites zone via Internet Explorer's configuration options.
Then please check if you have Static Content feature installed for your IIS instance.
This particular error also happens if you do not have Static Content feature installed for IIS7 server where Veeam Reporter server resides.
You should install Static Content feature for IIS to resolve the issue.
Instructions for installing Static Content feature for IIS7:
1) Open Server Manager console navigate to "Roles -> Web Server (IIS)".
2) Go to "Role Services" section and press "Add Role Services" link.
3) On new "Add Role Services" wizard window enable "Static Content" feature in "Web Server -> Common HTTP Features" section.
4) Go to next step and proceed with installation of the component/feature.
5) "Static Content" feature now should be available via Server Manager console in Web Server (IIS) Role Services section and should have status "Installed".
6) Restart IIS7 if required.
7) Open Veeam Reporter webpage.