Just ran into this same problem myself. For future reference to reset the certificate, connect a null modem cable to the ZD (115200,8N1) default login is admin/admin and run:
enable
config
certificate
restore
At any time you can press ? for help, this should then cause the Ruckus ZD to restore it's factory certificates and reboot. Why they just can't put this in the manual I'm not too sure (maybe it's there and I haven't found it).