This is one where we need to finish the conversation we are having when the complaint comes in. Question, Complaint, Question, Question, Question
It can be triggering when someone is complaining because we want them to jump to action and solutions. Complaining is a part of change and you shouldn't worry about it. If anything, it shows progress. Rejecting the question would be further away in the pre-change process.
I would validate the issue they raise, ask for more details, ask for how, why and when it happens so they have to really think it through and then ask the person in the conversation to ask for at least 3 solutions. If they don't have any, grab a couple of people there and then and brainstorm it out or bring it up in the team meeting and ask everyone to discuss it.
If you don't go into it deeply you miss the opportunity to call it for what is may be (nothing) or to learn from it.