Try moving the USB enable to after the initialization.

Code:
;    INT_ENABLE  USB_INT
ENDASM

;----[Initialise USB and Interrupts]----------------------------------------
    PAUSE 100                    ; Allow VUSB to stabilize
    USBINIT                      ; initialize the USB driver
    USBSERVICE                   ; service it once
    UIE = $7F                    ; enable USB interrupts
    UEIE = $9F                   ; enable USB Error interrupts
@ INT_ENABLE  USB_INT
Enabling before USBINIT probably wasn't a good idea on my part.

And while USBSERVICE is a PBP statement, the code is all ASM and does not use any of PBP's system variables. So change it back to ASM "type".