It is difficult for me to know what is working for you and what is not. When German is translated to English there is often confusion with the word "NOT", double negatives etc. Maybe you can tell me in English what is working and what is not working.
There are 3 basic methods that VC can interact with XBMC. Just because one method works does not mean that the other methods will also work.
Methods of communication from VC to XBMC are:
1) through the http interface (actions like XBMC.send and XBMC.execbuiltin)
2) through udp broadcasts (actions like XBMC.Btn.KB)
3) through keyboard emulation (sendkeys, inputkeys, and dxinput actions)
The first two should work reliably if you have set up everything correctly, and your firewall is not blocking VoxCommando from talking to XBMC through the network. Method 3 (keyboard emulation) will only work if XBMC is on the same computer as VC. Even then, it will work on some machines and maybe not on others.
In your command history image that you uploaded, it appears that your XBMC.send commands are not working (shown in red). As Kalle suggested to you, you need to roll your mouse over these red items in the command history window to see more information, or you need to enable logging and send us the file "voxLog.txt"
If you are not using a password in XBMC then it probably doesn't matter if your username is uppercase or lowercase, so maybe you are having a problem with your network or firewall.