I was getting this from podman on a CentOS 8 box:
"error from slirp4netns while setting up port redirection: map[desc:bad request: add_hostfwd: slirp_add_hostfwd failed]"
It was fixed by killing off all podman and /usr/bin/conmon processes as the user that I was running the commands as. Note: Don't do that as root using killall unless you limit to only your user.
The underlying error may have been running out of FD.