Cannot conect to SpeedTouch router using WPA

I have a SpeedTouch router which supports WEP and WPA encryption. Whenever I try to connect using WPA using my iMac, I get a consistent "Invalid Password" message, or I am prompted for my password over and over. Sometimes, the message is "Connection time exceeded". I'm positive the password is correct. I solved this problem by switching to WEP, which works fine (it always accept the password). A laptop PC connects using WPA with no problem. The version of the Airport Utility is 5.4.2

I've tried the solutions proposed at http://discussions.apple.com/thread.jspa?messageID=11036144&#11036144 and http://discussions.apple.com/thread.jspa?messageID=10955939&#10955939 but neither of them seems to work.

I would appreciate any suggestions