You changed the firstname.lastname@example.org for a vCenter Server system from the vSphere Client. Now you no longer remember the password, so you use the Copy icon on the Default vCenter Credentials page and paste the password into the VMware vCenter Single Sign-On Login Screen. The login process fails.
When you change the password for your SDDC from the vSphere Client, the new password is not synchronized with the password that is displayed on the Default vCenter Credentials page. That page shows only the Default credentials. If you change the credentials, you are responsible for keeping track of the new password.
Contact Technical Support and request a password change. See Get Help and Support.