I have tried to narrow down the cause of the issue. Since I wasn’t successful I ultimately escalated the ticket to my colleague and communicated to the client that if they can’t narrow it down either, we will ultimately have to do that.
Not a pretty way to resolve the issue but sometimes it is like it is :/

My ultimate goal with every ticket is to not just close the problem but to resolve the root cause for the long term. But that (sadly) clashes with the amount of resources a client is willing to spent.
Solve the issue within 5min that arises every 10 days or so or solve the issue with 2 hours spent on troubleshooting and eliminating the issue