Please check the release notes, due to some legal requirements on using default passwords, a change was made in 8.5 that if the default admin/admin is still present, or a new AP is deployed in 8.5 and later, the serial of one of the APs in the VC must be used as the password when logging in and then changing the default password to something speific (not the serial and not 'admin). This would have happened to you if the password was still 'admin'