I am not having a problem. I just thought I would tell about some simple experiments that I tried.
I recently started playing with a PIC18F25K20. The little development board I am using has a 3-pin socket into which a crystal can be placed which permits easily changing crystals. Oddly, the middle hole of the crystal socket is not grounded as I thought it might have been since that would make it readily able to accept a resonator in place of a crystal. The board does include a 15pf capacitor from each side of the crystal socket to ground so maybe having the middle hole grounded would result in a bit too much capacitance. The intended clock frequency in this system is based on using a 16 MHz crystal which is scaled up via a PLL internal to the PIC resulting in a 64 MHz clock.
In any case, after playing with a couple of different crystals (16 MHz and 4 MHz), I started poking around in my stock of crystals and resonators to see what else I might try. I tried a 16 MHz resonator and found that it worked well anyway. Of course, the capacitors on the board were still present so not having a ground for the capacitors built into the resonator did not seem to matter much. Next, I tried a 20 MHz crystal which worked and so the PIC must have been over-clocking at 80 MHz for the brief period that I let it run.
Then I became more adventurous and tried a 3-pin 10.7 MHz ceramic filter from the I.F. circuit of an FM radio. That worked. I tried a 2-pin 4.5 MHz ceramic filter from an old TV. That worked. I tried some 2-pin crystals from 49 MHz walkie talkies which operate at 3rd overtone so the crystals are actually around 16.5 MHz. They worked. I tried a 2-pin 455 KHz ceramic filter from an AM radio circuit which did not work, nor did a 32,767 Hz watch crystal.
Of course, I know that I was pushing the PIC in ways for which it was not intended. For one thing, I never tried changing the Config settings related to the oscillator. I was just poking crystals into the socket to see what ran. Has anyone else had similar experiences with the use of odd crystals or resonators?