This just seems like a common problem, however I was not able to find a solution on the net. For some reason Ubuntu doesn't like WPA/WPA2 personal encryption. I can connect to the network fine (others have reported they are not able to connect and the 'Authentication Required Window' keeps on appearing), however I am unable to connect to the internet.
When I change the security settings of the router to 'Open' or to 'WEP' I can connect fine. However 'WEP' slows down the connection by a factor of 2.
Several people have suggested to use 'wicd' instead of 'network-manager' I tried that but I got the same problems.
Please note that I have had this problem with Ubuntu 11.10 and now it seems it has carried over to 12.04.
Also note that I dont think it is a problem with the wireless router (Medialink) because my Windows 7 machine, iPad and Android phone all connect fine.
Any ideas? Thanks in advance