Today's hard-won tip, after struggling for a week.
I couldn't smb-mount one server on another. I've been upgrading all my servers to the latest Fedora 27, but I didn't upgrade one of them, because ... I'm chicken. If that server fails I'm up to my neck in deep water.
I tried rebooting it. I tried using nfs instead of Samba, but that gave me a readonly mount, and I couldn't work out why.
Eventually, I added ",vers=2.1" to the options, and that did the trick. It seems that Fedora 27 tries to use a version of the Samba protocols that Fedora 22 can't understand, and instead of saying "Oh, OK, I'll use an older protocol then" it just fails with the obscure message "mount error(5): Input/output error".