Today I upgraded our ZyXEL NAS to firmware V5.21(ABAG.7). After the upgrade, I was unable to into the web interface. SSH login for admin worked fine.

Because of Zyxel security advisory for the remote code execution vulnerability of NAS products — Zyxel Community the password no longer may contain special characters. If you used those special character in your admin password before update, you will need to reset it.

You should be able to reset admin password using the RESET button as stated in the documentation. But that did not worked for me.

If you have admin SSH access to the NAS, you may change the password by executing smbpasswd command. Web interface uses Samba for the authentication. As soon as you change the password using smbpasswd you should be able to login to web interface. Finally change the password in the web interface to make it permanent.