Hi,
The USB_enable() for MSP430F550x don't switch on XT2 correctly:
#if defined (__MSP430F552x) || defined (__MSP430F550x)
GPIO_setAsPeripheralModuleFunctionOutputPin(GPIO_PORT_P5, GPIO_PIN2);
GPIO_setAsPeripheralModuleFunctionOutputPin(GPIO_PORT_P5, GPIO_PIN3);
#else
According to slas645i.pdf (Table 52 on page 84) here we have to set P5SEL.2 bit, to enable XT2 on shared pins.
Regards,
Juris