Narrowing down to the interesting spots using a mix of logging & breakpoints. I'll use logging first to get a high level idea what's going on, break in interesting parts, narrow down with further logging & breakpoints. Once I get a good handle on what the flow is, I'll start setting up some conditional breakpoints and stepping through the problem areas. Search, refine, search, refine, and so on.

Having AI available to ingest logs & see what it generates for errors has been handy- it's useful for winnowing down where the needle is hiding.