I have also had this problem, and believe it originates from using Norton Identity Safe across Chrome & Safari.
I'm speculating here, but in my case the set-up was that Norton Identity Safe apparently used my login keychain for storing UUIDs, which (again, speculation) the NIS software would then look up in its encrypted online database and retrieve the valid website username & password.
Since setting up an iCloud account, I notice that new usernames and passwords get saved in my iCloud keychain instead.
As I was confident that all the user and password credentials I need are now in my iCloud keychain, the solution for me was to open Keychain Access and select login keychain at the top left, then filter the list and delete out the offending entries. These would be the ones marked as Kind 'Internet Password'; the corresponding credentials in iCloud keychain would be of the Kind 'Web form password'.
Hope this helps!