Seeking Help with Debugging Unhandled Fiber Termination Errors

I'm having trouble with fibers. I'm getting a "Fiber terminated with an unhandled error" and I don't know how to properly debug this. Setting breakpoints does is not helping, as I'm just landing somewhere in some starve methods in effects internals.
I've boiled down my code to some playground example. Would appreciate some pointers on how to debug this in general.