Update to El Capitan, System preferences, iCloud continues to ask password

iMac late 2009.

After updating to El Capitan, on system preferences, iCloud tab remains with "insert password" also after inserting it.

It isn't a continue request but simply when accessing to preferences is not possible to change options.

Already tried to log out from iCloud, restart, log in again.

It access during setup and it syncronize all than notes and do not permit to access to "iCloud Keychain".

Already tried to reset PRAM.

Already tried to delete files under library/Application Support/iCloud/Accounts

Accessing from accounts tabs (instead of iCloud tab), it seems to enter normally, but only changing account and returning to iCloud one it asks again password.

Furthermore when trying to activate iCloud Keychain it doesn't work for "unknown problem of connection".

other notes:

on my macbook mid 2009 I haven't this problem (updated to El Capitan)

on my iPhone and iPad no issue too

on web iCloud no issue

accessing with different iCloud account ... IT WORKS!

So I believe is something related to my account but only on my iMac, don't know if some cache has to be deleted.

Thanks to all!

Andrea