Other device connection shuts down Arduino

I connect my Leonardo as a USB device on a Windows 8.1 Laptop. I use it to control software which is connected to my guitar amplifier also via USB. The arduino is powered via USB; the amp's processing unit is wall-powered.
This setup worked seamlessly until a few days ago when all of a sudden, the following is now happening:
EITHER

  • I connect the arduino only first and controls the software. As soon as I connect the amp as well, I get no response from the Arduino now. OR
  • I already have the amp connected. When I connect the arduino, Windows replies with 'USB Device Not Recognized'.

I'm not sure if the arduino is on/off when it's 'not working' since it's hidden inside casing etc which I still have to dismantle. I'm just asking to see if anyone else has this problem when another device is connected to the same laptop.
Again, this used to work perfectly until a few days ago, irrelevant of the order in which I connected Arduino and amp. So this maybe could be caused by a Win8.1 update, maybe?

Seems to me the laptop is just having trouble powering all the USB devices. Laptops often have limited power for USB, especially when running from the battery.