Using "tmutil set destination -p" fails with a failure to save the password to the keychain

I've got a new FreeNAS setup, and I'm trying to get Time Machine backing up to it from both of my household's laptops. I've set up an AFP share for one of the datasets on the NAS and have got my laptop backing up automatically after solving an issue supposedly related to mDNS.

However, applying the same workaround that I used for my laptop to my girlfriend's doesn't quite work:

Richelles-MacBook-Air:~ richelle$ sudo tmutil setdestination -p afp://richellemba@macknas.local/Time\ Machine
Password:
Destination password: 
2015-03-21 23:41:10.889 tmutil[416:9720] ### syncProxyWithSemaphore error:Error Domain=NSCocoaErrorDomain Code=4097 "Couldn’t communicate with a helper application." (connection to service named com.apple.systemadministration.writeconfig) UserInfo=0x7f9ca3706660 {NSDebugDescription=connection to service named com.apple.systemadministration.writeconfig}
Failed to save password to system keychain. (error -1)
The backup destination could not be set.

Further investigation into the above error has so far turned up a blank, and I'm stumped. Does anyone have any ideas?

The laptop in question is a 2014 MacBook Air running 10.10.2.