Noghar " Is this something to do with the new Virgin/O2 ID system? "
No, it was way before that.
I cannot give you an exact date but I think it was late 2023 when VM made changes to the authentication and security of VM webmail and My Virgin Media accounts. The VM02 ID change merely builds on and extends that basic concept.
The way they introduced it back then was that if you wanted to make any changes to your VM account, resetting the password for example, you were forced to specify a non VM email account (Gmail, Outlook.com, Proton etc) to link with your VM account.
That non VM email account was then used by VM to send verification codes / links and that non VM email address also became the username you had to specify in order to sign into your VM account. In other words, the non VM email address became your "Virgin login".
" for the last twelve hours my app hasn't been able to log into the email server "
The problem you are seeing with your app is almost certainly caused by the other change that VM made quite a while ago. You now need to use a VM "mail app " password in the settings of your mail app rather than the password you use to sign into your webmail or My Virgin Media account.
The problem is to generate that you need access to the My Virgin Media account for that VM email account.
I had better leave it there for the time being.
Coenoby