I had fixed the problem:
Recreated folders for each project in the reporting server and uploaded the reports with the tool "TFSUploadReports.exe" described here:
To upgrade the default reports for a project
1. Open Team Explorer or Visual Studio 2005 with Team Explorer.
2. On the Tools menu, click Connect to Team Foundation Server.
3. In the Connect to Team Foundation Server dialog box, select the Team Foundation Server you installed in Step 7.
Note If the drop-down list is empty, click Servers to manually enter the settings for the Team Foundation Server you installed in Step 7.
4. Under Team projects, select one or more team projects from the list. Team projects with a check mark next to them will display in Team Explorer.
5. Click OK.
6. On the Team menu, point to Team Foundation Server Settings, and then click Process Template Manager.
The Process Template Manager lists each process template installed on the Team Foundation Server.
7. Click Download.
8. Download all the process templates you want to use in Team Foundation Server RTM (including your customized process templates and the default process templates for your project types) and save them to a specified folder.
9. Click Close to close the Process Template Manager.
10. At the command prompt, change directories to the directory where you installed the upgrade software.
11. At the command prompt, run the TFSUploadReports tool by typing the following, where projectname is the name of the project you want to upgrade, servername is the name of your Team Foundation Server, drive is the drive where the templates are stored, templatepath is the path of the template directory, and templatedirectory is the name of the directory where the stock report templates are stored:
TFSUploadReports.exe /s servername /p projectname /t drive:\templatepath\templatedirectory
12. Repeat these steps for each project in which you want to use some or all the default reports.
That solved the Problem.