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@Gordon
I'd connect the probe between GND and xout (xout is amplified by the CPU so won't matter as much). If the crystal is on the edge of working adding a probe might kill it - but then if it's running well enough you'd be fine.
I'd say that yes, connecting to ground might cause problems. I haven't seen that before, but I'd guess it isn't supposed to be grounded.
Also, my impression was that the capacitance needed for the crystal is the roughly the load capacitance minus the board capacitance of maybe 2pF. 27pF looks way too high, and it seems you'd be better using something nearer 4pF.
It's why I gave up with the Pico and used a resonator that has the caps inside it :)
It's a total black art (at least to me) but often you can try different values and see how strong/well shaped the oscillations look on the scope.