postgres: archive_command can't write to an nfs share
You are probably a victim of selinux or something like it, in which the same system OS account can have different permissions depending whether it is running as a deamon or from a command line (and, it often seems, depending on the barometric pressure, the phase of the moon, and what you had for breakfast).

You can turn it off, or put it into permissive mode where it allows the access and just logs a gripe.