Delete the keychain associated with that website. It may be corrupted.
Go to Safari > Preferences > Passwords. Find and delete the website in the list.
Now open Keychain Access located in HD > Applications > Utilities
Select Passwords on the left.
Now type the name of that website in the search field top right corner of the Keychain Access Window.
Then right or control click the idmsa.apple keychain then click Delete.
Quit and relaunch Safari then go to Safari > Preferences > Passwords and make sure AutoFill user names and passwords is selected
Now go to that website and enter your login data. You should be prompted to save that data to a new keychain.