I thought they were referring to the inability of the aliens to make an accurate calendar.
But I thought that the Trisolarians' inability to predict their planets' path also seemed weird. Yes, the three-body problem isn't analytically solvable, but you can brute force it, and I would imagine that a species that is capable of creating sophons and starships with hulls made out of the Strong-force could calculate their planet's trajectory for the next thousand years without too many problems. (It would be academic, anyway - if you can build that kind of technology, why do you even need planets?)
And I also hated the sophons.
This kind of iterative process might yield some insights on how uncertainty creeps into the system.
The “we need to solve this problem!” thread was a ploy to get earthly scientists invested in their plight such that they could convince them to support their cause from the inside.