May I offer some practical advice?

You do have extensive logging, especially around the buggy code? Do you have source-level debugger that shows your variables and that can step/trace through code?

These are the 2 tools that I have found most indispensible when squashing bugs in the past. Somehow source-level debugging seems to have disappeared from the developer's toolbox the last couple of years. I am told you don't need it anymore as you can write tests (???).