ZFS does not appear to like names with spaces in.
If this is the case, the web GUI should prevent it as a validation error.
The problem occured when I tried to create a pool with the name "external backup". The web gui accepted this, but when I tried to apply the changes I got the dreaded "Error: The changes could not be applied (error code 1)" error message. After thia has happened it does not appear that there is any way back. ZFS appears to continue to work, and I have been able to delete, the original pool, and create a new one. But I still get "The configuration has been changed." everytime I go into the ZFS pool screen.
The error log looked like this:
zpool create -m /mnt/external backup external backup /dev/da1
cannot open 'external': no such GEOM provider must be a full path or shorthand device name