The External User flag for admin user has set when Ldap was activated. Even disabling Ldap this flag doesn't disappear.
Check from the JMX Console, UCMDB:service=URM Services, we could see the 'isExternal' flag set to true looking at: listResourceTypes then clicking on Auth_USER.
Edit this and set it to 'false', to resolve ths issue.
But if the password was corrupted, delete the user from UCMDB: UCMDB:service=Authorization Services, removeUser, then created it with 'createUser', then added the SuperAdmin role with 'grantRoleToUser'. Now the user should work.