Ok, I found another thread where Melanie mentions 17.2.2.8:

"The PIC18FX455/X550 devices have a built-in 3.3V regulator
to provide power to the internal transceiver and
provide a source for the internal/external pull-ups. An
external 220 nF (±20%) capacitor is required for stability."

I don't know why people are mentionning 400nF when the datasheet states 220nF, give or take 20%. Either way, I don't have capacitors in this range so I'm stuck for now.

Robert