We had a similar issue recently. We used the steps to reset the SSO admin password and unlock the account. We can now successfully login via the web client using the admin@system-domain account.
What isn't working for us is the upgrade of vCenter. Starting with upgrading SSO it asks for the admin password which we now have. When I enter that password it gives an error that it's blank or incorrect. Is it really looking for the admin password or something else?