It's a case of “it works on my laptop but fails in the CI (AWS + Docker)” ;)

In the container all calls fail with:

Unix-error: Function not implemented (function: "clone3", argument: "")

This is the host's info:

$ uname -a
Linux 770143d8f119 5.15.0-1031-aws #35-Ubuntu SMP Fri Feb 10 02:07:18 UTC 2023 x86_64 GNU/Linux

If I replace Eio_main with Eio_posix it seems to work.