Ok, let's just go back to the original demo without any alteration:

Code:
' USB sample program for PIC18F4550 CDC serial port emulation
'  Requires PBP 2.60 or later

'  Compilation of this program requires that specific support files be
'  available in the source directory.  For detailed information, see
'  the file PBP\USB18\USB.TXT.

ASM
CONFIG CPUDIV=NOCLKDIV ; CPU runing at full speed
CONFIG USBDIV=OFF ; Low speed USB clock diviser disabled, not used here
CONFIG FOSC=HS ; Use of High Speed crystal oscillator
CONFIG PLLEN=ON ; Enable PLL multiplier, 12Mhz oscilator x 4
CONFIG PCLKEN=ON ; Primary clock source is enabled
CONFIG FCMEN=OFF ; Failsafe clock off
CONFIG IESO=OFF ; Oscillator switchover mode disabled
CONFIG WDTEN=ON ; Watchdog timer on
CONFIG WDTPS=512 ; Watchdog divider is 512
CONFIG MCLRE=OFF ; Disable MCLR pin, enable RC3
CONFIG STVREN=ON ; Stack full/underflow will cause reset
CONFIG LVP=OFF ; Low voltage programming disabled
CONFIG BBSIZ=OFF ; 512kW block boot size
CONFIG XINST=OFF ; Extended instruction mode disabled
ENDASM

Include	"cdc_desc.bas"	' Include the CDC descriptors
INCLUDE "USB_ASM_Service.pbp"  ' USB Init and Service interrupt routines

'Define pin function
TRISA = %00000000 'PortA all digital output
TRISB = %00000000 'PortB all digital output
TRISC = %00000000 'PortC all digital output

ANSEL = %00000000 'Disable analog input on other pins
ANSELH = %00000000 'Disable analog input on other pins

'Define weak pullups
WPUA = %00000000 'No weak pullup on portA
WPUB = %00000000 'No weak pullup on portB
    
Define  OSC     48

buffer	Var	Byte[16]
cnt	Var	Byte

'Pin function declaration

Pause 10

'	USBInit			' Get USB going

' Wait for USB input
idleloop:
'	USBService		' Must service USB regularly
	cnt = 16		' Specify input buffer size
	USBIn 3, buffer, cnt, idleloop

' Message received
	buffer[0] = "H"
	buffer[1] = "e"
	buffer[2] = "l"
	buffer[3] = "l"
	buffer[4] = "o"
	buffer[5] = " "
	buffer[6] = "W"
	buffer[7] = "o"
	buffer[8] = "r"
	buffer[9] = "l"
	buffer[10] = "d"
	buffer[11] = 13
	buffer[12] = 10
	buffer[13] = 0

outloop:
'	USBService		' Must service USB regularly
	USBOut 3, buffer, 14, outloop

	Goto idleloop		' Wait for next buffer
... does not work.

I must say that after 2 days of digging the forum and the internet I am starting to get clueless about what to do to solve it. Is there a timer or an interrupt register to set up manually in order to run it properly? I could try to run it on a 18F4550 board I have but that won't solve the problem for my 13K50 project.

Also, I found out that USB_ASM_Service.pbp does not work in conjunction with INDT_INTS-18.bas and SPWM_INT.bas from Darell. I will need at least the latter, so maybe I should find another solution than actually trying to use USB_ASM_Service.pbp.

Any idea for troubleshooting this? or pointing to an interrupt solution that I could use? I'm starting to get clueless. Any help greatly appreciated!

BTW, I'm running PBP 2.60 in MCS 4.0.0.0, in conjunction with MPLAB 8.53.