fmbsg wrote:What I find confusing is, that it was possible to flash with USB2 yesterday and since today, the brick is only visible on USB3. I checked all ports: It disappears on all USB2 ports and it shows up on all USB3 ports.
Well, the obvious thing to try here is to flash the original Lego firmware to the NXT. Does it then show up on the USB 2.0 port again? This is possible with nxjflash. Open a command prompt and execute nxjflash -b /path/to/legofirmware. The lego firmware binary can be downloaded from Lego. Note that you have to unzip the firmware first.
If that sounds to dangerous to you, simply manually put the NXT is firmware update mode (the mode, where it starts ticking) using a paper clip or so and connect it to the USB 2.0 port again. Does it then show up in the device manager? While the brick is ticking, you can flash leJOS back again at any time.
BTW: can you tell us more about your hardware? I.e. what's the chip driving the USB 2.0 and USB 3.0 ports? Which USB 3.0 driver is installed?
I could imagine, that the USB 3.0 driver messes with some parts of the Windows USB stack. After all, USB 3.0 is not natively supported by XP.
Do you have Windows 7 by any chance?