hd_scania your Atheros NIC may be too recent for ath(4) to support IEEE 802.11n, hence you may want to try force your access point to fallback to 11g, by doing:
$ sysrc create_args_ath0+="mode 11g"
$ service netif restart
See if this helps...
PS: What chipset are we talking about? Is this a fresh installation, or did the problem arise out of the nowhere, possibly following an upgrade or a rollback?
EDIT: on FreeBSD the networking meta-service is called 'netif' and not 'network'; the latter is a NetBSD_ism