@Kevin_Z 

Hi, thanks for the reply.

The firmware should be the last available for this router.

Anyway, after a few more resets/tests I tried to access using Chromium instead of Firefox and it worked.

Before the update I was able to access the management interface with no problem using Firefox, that's why I didn't think to try another browser before.

Funniest thing is that I tried with Firefox again after Chromium, and it worked too.

Nonetheless, both the browsers showed various (minor) issues in the correct visualization of the interface (e.g  the Internet Time and UPnP pages weren't available, the left panel/menu wasn't shown correctly).

I managed to set the various parameters, all but the passwords for the "user" and "support" accounts which aren't available anymore (the drop-down menu in the Access Control Passwords page shows only "admin").

As I am writing I tried again to access with Firefox and it worked flawlessly (still "user" and "support" accounts aren't there anymore).

My conclusion is that the last firmware is a little buggy and the interface works unpredictably.

Hoping that I won't be locked out again, is there a way to fix the "user" and "support" accounts?

(although from your reply I suspect the way would be to leave the router with its default settings).

Thanks for your help.

(I forgot to mention in my first post that I am connected to the router via Ethernet cable and I don't use wireless (i.e. it's disabled)).