Login Loop

Suddenly this morning whenever a user tries to log in to my site it just sits on the login page. I have deactivated all plugins and the same thing happens. If I switch to a default wordpress theme I can log in fine. I’ve seen several references to this issue on the forums, but no answer. Can anyone help with this. This is what I am doing:
When I type in the address: https://www.domain.com/wp-admin
It redirects to: https://www.domain.com/wp-login.php?redirect_to=http%3A%2F%2Fwww.domain.com%2Fwp-admin%2F&reauth=1
in Firefox. In Chorme and Safari it just shows: https://www.domain.com/wp-login.php