After loading the phone over the usb-port several hundred of times the mini-usb-port of the phone is worn and should be changed. If you do some research you'll find out that the usb-port can allegedly only be changed by replacing a complete board. This spare part can be bought for about 80 € at ebay which is far too much if you ask me. If anybody has an idea how to change the mini-usb-port without replacing more than that, this would be fine.
Thanx folks and bye
Chris