Arduino Mega 2580 - unable to program after Test Sketch

The situation:

The board worked well, until i've loaded the example sketch "SerialPassthrough" (unchanged from original). The program seems to block the serial port after restart, and therefore reprogramming of the board is not possible.

The description says, that the boot loader waits some time (? how long) before activation of the loaded sketch. This seems not to be true. Even resetting the board before download does not correct the problem.

However, ist seems that the USB interface ist not able to activate the bootloader again.

How is it possible to remove/revoke a loaded sketch (aka reset the bootloader) to its default state (without using the STK500 ISP programmer to erase the chip)?

Any hints/ideas?

Thanks in advance!