Sometime between 5.3 and 6.3, ccmp became the default cipher. If your router is confitgured for tkip, you would also see a previously working system fail to connect.
If this is the case, ccmp is deemed more secure so the best fix would be to set the router cipher to ccmp. Otherwise you can use a wpacipher entry in your /etc/ifconfig.ath0 as described in
ifconfig(8).