I introduced a nasty bug that notably affects dealing with processes with c62b20ec66a8371e4a2e6ca919fc124b70bdacaa, pushed March 7. If you built from the git source after that, please pull and rebuild. The issue is fixed with 2d88c99db1d1ba841fc221cab0ca6ea188872f2e.
The versions in between use dup() from an invalid file descriptor, typically causing the new process to be connected to invalid streams and report I/O errors (which typically fails as well because stderr is not correctly connected 
).