So in The Angels Take Manhattan, River says it would be impossible to land a Tardis in New York during the 1930s. “This city’s full of time distortions. It’d be impossible to land the Tardis here. Like trying to land a plane in a blizzard. Even I couldn’t do it.” Then, later, after the Ponds create a paradox that destroys (most of) the Weeping Angels, the Doctor says, “I can’t ever take the Tardis back there. The timelines are too scrambled.”
I mean, yes, there’s legitimate criticism of this on strictly logical grounds. Why don’t they just take a bus to New Jersey? But, you know, timey wimey. The time distortions are entangled with the Ponds’ timelines. Their biodata can’t be extricated from it. You can always gobbledegook your way out of time travel stories. The point is emotional: it’s a callback to Amy’s Choice, and Amy made her choice once and for all.
In The Return of Doctor Mysterio, River has just gone to meet her destiny in the Library. The Doctor, lost and grieving after living with her for 24 Earth years, does what we should have expected him to do: he scours the universe for a magic wishing stone that, when mounted in whatever jury-rigged timey-wimey monstrosity he’s cobbled together, will hopefully undo the time distortions created in The Angels Take Manhattan.
“It’s a time distortion equalizer thingy. There’s been a lot of disruption in New York—my fault actually—hopefully this will make it all calm down.”