The original bug report failed to explain how to trigger the bug.  I think
I know of a simple way to trigger the bug, but have not yet created the
simple test-case.  I believe that the bug can be triggered like so:

Create some medium-sized "random" (arbitrary) snippet of guile code. Send
it to the guile repl server.  Do it again and again, as fast as possible.
The bug will trigger.

By "arbitary/random" I mean some code that is simple but non-repeating:
say, concatenate some random strings, count the number of letters in them,
take the square root -- just do assorted, non-repeating computations that
force the REPL to evaluate some new blob of scheme code every time.

Now, once out of every 5 or 50 times, introduce a bug in the scheme code,
causing the REPL to throw an exception.  Using ut8 strings might also be a
required ingredient.

I believe that this will trigger the bug. I believe that the bug is some
unprotected section in the guile interpreter, and a race condition clobbers
the guile stack(s) and it all goes downhill from here.