Your question is Owning a Critical Troubleshooting Escalation. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Tell me about a time you had to troubleshoot a complex software issue for a customer where the root cause was not immediately clear. Walk me through how you diagnosed it, how you communicated with the customer during the process, and what the final outcome was.