Thanks! Nope, that's not it: my girlfriend's phone has never been configured in the router for sure, but it received a random IP from the DHCP address pool as soon as she put in the passphrase.

Resolved my issue with the N9 by assigning a static IP in the router. Pretty tedious to have to log in to the router, remember where to set static IPs, type in the MAC address of the N9 etc. etc.

Better than no WLAN connection though...!