Two particular notes around this (exact commands assuming gdb, but other debuggers should have equivalents):

Pass various arguments to `backtrace` rather than just relying the default. Chances are it will have some non-optimized-out variables, which you can use to figure out what's going on.

Use `info registers` and see what looks like a pointer, then cast it to a type you suspect it is. Note that this can be done for any stack frame.