Just a guess here, but 2 things jump at me.

INTRC is not a valid choice according to the datasheet, is this a typo or something strange.

Assuming the above is just strange to me, try setting SCS bits to SCS1=1 and SCS0=0. This will force it to use the internal no matter what the config says (maybe).

Now if this makes it work, I will have no idea why. My guess there is, well, hmmm, maybe, no no, ummm, clearly I have no idea. SRY