Did you already read this sticky ?
300 Mbps connection requirements! Please read! Set your wireless to WPA2 with AES encryption if possible.
Sometimes windows remembers the settings of a connection even if you change them.
There are two ways you can try to get your 11n connection back.
1. Delete you Homenet profile in the wireless utility, then let the card detect it again and fill in your passphrase.
If this does not help...
2. Change the SSID in the router to a never used before SSID with encryption matching the 11N requirements and connect to it. This "new" connection should be recognized correctly by the client.