Ah, the good ol’ Proven Product Syndrome.
I’m sure we’ve all had the experience of chasing down a mysterious bug and, when the cause is finally located, thought “how did this ever work?”
It can be hard enough to track these things down with full access to & details of the system - let alone remotely as a stranger.
You seem to be taking the right sort of approach - I can’t think of anything else obvious to suggest.
Good luck!