USB_Init() no longer calls sei() to enable global interrupts - this must now be done in the user application once all init code has run.

Demos/Device/ClassDriver/VirtualSerial/VirtualSerial.c

@@ -76,6 +76,7 @@ int main(void)
 	CDC_Device_CreateStream(&VirtualSerial_CDC_Interface, &USBSerialStream);
 
 	LEDs_SetAllLEDs(LEDMASK_USB_NOTREADY);
+	sei();
 
 	for (;;)
 	{

Demos/Device/ClassDriver/VirtualSerial/VirtualSerial.h

@@ -40,6 +40,7 @@
 		#include <avr/io.h>
 		#include <avr/wdt.h>
 		#include <avr/power.h>
+		#include <avr/interrupt.h>
 		#include <string.h>
 		#include <stdio.h>