Strange problem. I can run pretty much the same thing you have here on an 877A and it
works fine.
Code:
DEFINE OSC 20
@ DEVICE HS_OSC

DEFINE ADC_BITS 10
DEFINE ADC_SAMPLEUS 50

B0 VAR BYTE 'just used for SERIN input and testing
adcVal VAR WORD 'used for ADC value
W1 VAR WORD 'used to test DEC function in SEROUT2

TRISA = 255 'PORTA is all input
ADCON1 = %10000000 'PORTA analog, no ref voltages, output right justified

  PAUSE 500
  W1 = 1022 'just to test DEC
  SEROUT2 PORTC.6,84,["Ready to go ", DEC W1, 10] 'Verify communication
  pause 10

loop: 
  SERIN2 PORTC.7,84,[B0] 'Read in a character on PORTB.2
  PAUSE 10
  Serout2 PORTC.6,84, ["Getting ADC value", 10]
  pause 10
  ADCIN 0, adcVal 'perform ADC and store 10 bits in adcVal
  pause 10
  SEROUT2 PORTC.6,84,["ADC value: ", DEC adcVal, 10] 'Send the decimal value
  Goto loop 'Start again
  
  END
On a LAB-X1 board it reads the POT on RA0. Changing to ADCIN 1 it reads the POT on RA1.

What happens if you disable A/D and try blinking an LED on RA0?