Considering the discussion on "Unanswered questions reminder" and some flags on such questions, I am proposing a more clear close reason for questions that cannot be answered:

Too specific or impossible to answer or to replicate. Generally questions about extensions where the software developer is the only one who can offer a solution are off-topic because the community can not provide an answer to the problem.

While not directly mentioned, clearly the time is also a factor. So this close reason should not be used in a first instance, but rather after a decent period of time in which the OP is expected to provide more details or to answer any addressed questions.

At least one moderator (apart from me) is required to review this close reason.

Any feedback is of course welcome.