I'm using Mega4809 Curiosity Nano in circuit with extern power supply. VOFF is grounded.
Kit unchanged out of the box, all recent software updates are installed (AtmelStudio7, nEDBG).
After PowerOn I can program/debug using nEDBG via USB without problems.
I plug the USB cable out and in a few times and everything works every time - but then suddenly
and permanently I get the message "nEDBG disconnected". Interestingly programming via
drag and drop to curiosity nano storage disk is still possible!
The nEDBG blockage can only be released by switching off/on the external supply voltage.
Tested with two kits.