This question is actually rather difficult, because it's not asking what is CAUSING the issue, it's asking how you would actually TROUBLESHOOT it. How do you start? How would you approach a client, who may, or may not be technically literate, but who has an issue like this? Be creative, and ask questions about how long it has taken place, have them read the error to you, ask when exactly the error happens; remote in, and look at the error log for yourself, ask them to reproduce. All of those are VALID ways to START to troubleshoot it. If you cannot answer this, then the world of indepth desktop/software support is NOT for you.