I used Webmin to review previously manually created NFS eports on MacOSX High Sierra. Those contained an escaped blank "\ ". The exports themselves work fine. The directory path is truncated, the part after the blank appears in the clients column. Maybe a more general quoting problem? Under MacOSX is is quite common to have file and directory names quoted (", ' or ) in config files.
Are you running the NFS server on OSX, or exporting a directory to OSX?
I am running NFS as server on the same machine as webmin. Exporting folders like "/Users/username/Music/iTunes/iTunes Media/Music", quoted with a backquote before the blank character in etc/exports. Clients read from the export just fine.
Which NFS sharing module in Webmin are you using there?
After starting Webmin I click on Networking and then "NFS Exports"
Ok I see now - this will be fixed in the next Webmin release.