PIC24 PLL initial value
Please refer to section 2.7 in the datasheet. 


  If the PLL of the target device is enabled and configured for the
  device start-up oscillator, the maximum oscillator source frequency
  must be limited to 4 MHz &lt; FIN &lt; 8 MHz to comply with device PLL
  start-up conditions. This means that if the external oscillator
  frequency is outside this range, the application must start-up in the
  FRC mode first. The default PLL settings after a POR with an
  oscillator frequency outside this range will violate the device
  operating speed.


Looks like you will have to start-up with the internal oscillator and then switch to your specific settings.