Zyxel Router VMG8823-B50B - User password is not valid anymore after saving backup and restore

I've a router Zyxel VMG8823-B50B (Infostrada branded, Firmware 'V5.13(ABIU.0)b5_20190308').

I've configured it for remote (WAN) access, and after some troubles with applying some settings changes (another post has been opened for this issue), I've decided to try and trick the router by downloading the backup configuration, changing manually the settings I required and then uploading the new configuration.

I've changed nothing in the "Account" field, hoping that it wouldn't have screwed anything up with the remote access. Below is what I had and loaded (changing something for security reason).

Problem is that now I can reach the login page of the router from WAN, but I cannot login anymore, neither with the previous password nor with the clear-text exactly as reported in the "Password" field below.

Any suggestion?

{
"Enabled":true,
"EnableQuickStart":false,
"Username":"my_wan_username",
"Password":"_encrypt_here_some_base64_characters=",
"DefaultPassword":"_encrypt_here_the_same_base64_characters=",
"PasswordHash":"",
"AccountRetryTime":3,
"AccountIdleTime":300,
"AccountLockTime":300,
"DotChangeDefPwd":false,
"AutoGenPwdBySn":false,
"Privilege":"_encrypt_here_some_other_base64_characters=",
"RemoteAccessPrivilege":"WAN"
}