For now, I disabled the blur (to be honest, it is still calculated, just not visible...), added a fourth depthmap and did some other minor changes.
This is what it looks like:


There are still some artifacts, but most should disappear after some blurring and the rest gets hopefully hidden by the shading.
I also uploaded a published version of the shadows and would like to get some feedback on strange behaviour like shadows nearly everywhere and something like that. If you encounter any other problems than a bad fps I would like you to post a screenshot of the problem and some information on what graphics card you are using. Thanks.
Download it here: [url=www.slindev.com/files/projects/SunShadowsV1.7z]www.slindev.com/files/projects/SunShadowsV1.7z[/url]

Lite-C Tut. 1 Lite-C Tut. 2 Lite-C Tut. 3 (en)
Shader Tutorial (ger)

http://www.slindev.com