NAS Login Page No Longer Displays

[suitej]

I'm no longer able to connect to my NAS through a web browser.

The first error is "uses an invalid security certificate. The certificate is not trusted because it is self-signed."

This is nothing new; it's always done this. I always click "Accept the risk and continue", which normally shows the login page.

However, I now get "The page you are trying to view cannot be shown because the authenticity of the received data could not be verified." This is new.

Unfortunately my NAS is set to use HTTPS only.

I have tried various versions and editions of browsers and they all show variations of the above.

I have also tried to connect using Qmanager on Android but this claims "your login credentials are incorrect or the account is no longer valid." Nothing has changed since the last logon, some time ago now.

I don't want to reboot my NAS because the volumes are locked; I need to be able to login to unlock them! At the moment I can access my files and I want to keep it that way.

Can I install a certificate on my system to accept QNAP's self-signed cert?

Do you have other suggestions?