> I recommend first doing a capture of these functions first

I don't think that'll be necessary - the logging output ought to be enough. Capturing or redirecting those functions would be great if the debugger was working, but since that spoils it, it's not.

> Does that point to anything we can use to solve this?

It would be great to know what's calling FlsGetValue there. Obviously something within the CRT, since CPython doesn't use fibers anywhere. I guess it's probably some kind of locale settings that are only necessary when opening a file in text mode? But I don't know that we do that specially here.

Is there an option to capture full stack traces?