Fix Safari not logging in, or blank page problem
If you experience this issue you can download an alternate browser such as Chrome or Firefox, they do not have this bug.
If you want to keep using Safari follow these steps
1) Quit out of safari ( shortcut is command + Q )
2) Search for the 'KeyChain Access' application. (shortcut for search is command + spacebar)
3) Locate ANY item in the login folder that starts with com.apple.idms.appleid.prd..........
4) Delete any keys by pressing the Delete Key.
5) Re-open Safari and you should be able to log in normally
Why does this happen?
It seems there is a bug in the keychain and safari when you also have a linked iCloud account. iCloud is mistakenly trying to manage the security keys for your usernames and password and generates a bogus one that the system does not accept.