Hi,
there is a differenc between JTAG and UART: the JTAG-Adapter-Box has two interfaces. An UART-Interface that a serial port with TTL (3,3V) level that is the console port of the plug. I assume, that you are using this port.
The second one is the 8 pin JTAG-port. This special interface is for testing and debugging purposes and normally not used, when you are working as admin on the dreamplug. It gives you a hardware related access to the chips on the board.
For me: it is not rebooting, when I cut the USB cable to the JTAG-adapter box.
But I have some problems with the 230V power cable: it has a slack joint, so when moving the plug a little bit, the power goes away for a short moment and it reboots... Maybe this could the cause for your problem?
Regards,
Markus