Hi,
it is not important to have three exclamation marks in this text. The problem was i did some minor programm changes in a running system. Suddenly I was not able to write a new programm in the mega. I could not imagine that this change in a string could cause this problem. Therefor I checked everything, from powersupply to casing. I also ordered a second Mega 2560 (Freaduino) and was not deliverd in two weeks. I found an old usbprog in my "spare part stock". Which I have had to flash befor the first use. Finally I could programm the mega thru ISP (with three exclamation marks in source code).
This bug took me a lot of time and nerves. Nevertheless I am very satisfied with the Arduino Mega.
Konrad