I'm adding an extra answer as a complement to the excellent answers already present.
An XY problem seems to be a subset of the Einstellung effect, where a problem-solver gets stuck on a particular solution and is unable to backtrack mentally to see potentially superior solutions.
An example from Chess would be a person winning with a 5-move smothered mate in one game. The next game, the same player fails to see a faster 3-move mate because they are stuck on the idea of a 5-move mate. This psychological phenomenon affects everyone, novices and experts alike.
Specific to Q&A, the perniciousness of an XY problem comes from the fact that it is frustrating for everyone involved:
- The person asking the question asks the wrong question (which is related to their attempted solution rather than the original problem), and then finds it difficult to clarify the question because they are stuck on their proposed solution. The proposed answers are unsatisfactory because people are confused by the question or think the proposed solution is weird.
- People answering the question find it frustrating because the proposed solution doesn't make sense to them, since they are approaching the problem from a fresh angle and are (presumably) not being affected by the Einstellung effect, and they find it difficult to get the original poster to clarify their question.