After a extensive and nerve-wracking search for the error  that lastet at least 4 weeks, I found out that my problem is a bug of WSS3.0 when working in combination with IIS7.0 (Internet information services). Regardless wether you edited the max filesize in your sharepoint central administration you simpy couldn't upload files  bigger than 28MB.

By chance I found an article on Microsoft's support site  that pretty described my problem. To correct this error you simply have to enter the following xml section (under the <configurations> node) in the web.config
of your sharepoint site (found in \inetpub\wwwroot\wss\VirtualDirectories\):

<system.webServer>
    <security>
        <requestFiltering>
            <requestLimits maxAllowedContentLength="52428800"/>
        </requestFiltering>
    </security>
</system.webServer>

In this example the filesize was set to 50MB.
After I editet my web.config everything worked fine.

The corresponding link to the MS support article is:

http://support.microsoft.com/kb/925083/EN-US/

I hope that anyone else who has the same problem that I had will fix it faster than me.