After upgrade of the Symantec Endpoint Protection Manager (SEPM) to 14.3 RU5 administrators are not able to log into the SEPM with previous password that worked prior to upgrade.
Release : 14.3 RU5
As of 14.3 RU5, Symantec Endpoint Protection Manager (SEPM) lets system administrators create complex passwords for administrators that protect your computers against brute force attacks. You can set the password requirements to meet the password policy guidelines of your organization. These settings override the default SEPM password requirements.
After upgrade the secure password enforcement is enforced and the previous password no longer met the requirements and was using some invalid characters. The admin had to reset the password for the account ensuring that the password did not use certain unapproved special characters and met the other password criteria.
For more information about the new password requirements see document: Enforcing a secure password for administrators and client users