VM's email service is in a permanent state of crisis for many customers but, in fairness, a search on that error code and message did turn up a lot of references to some random Outlook behaviour from earlier this year.
I use Outlook 2016 on Windows 10 and, when I check/change the server settings, I do that with Outlook closed and I go via the 'Control Panel' route and the Mail option in the old control panel interface.
I find this much more intuitive than the 'wizard' type settings from within Outlook and it also means that Outlook is not running when I make the changes.
If you can, see if you can double check the server settings via that route, when Outlook is closed.
From the old 'Control Panel' and with the View set to large icons you should see an icon for Mail (mine says 'Mail (Microsoft Outlook) (32 Bit)' )
From the window that appears use the 'Email Accounts' button.
This should pop up a window 'Account Settings' and you should see the mailboxes you have configured. Select the mailbox you want and click the 'Change' button. It should bring up a 'Change Account' window
In the part for 'Logon Information' you use your full VM email address (not the third part address) and the password is your app password.
Then use the 'More settings' button to access the server settings. In the 'Outgoing server' tab you need 'My outgoing (SMTP) server requires authentication' with the option checked for 'Use same settings as my incoming mail server'.
On the 'Advanced' tab you use the server settings below
https://www.virginmedia.com/help/broadband/manage-email-account
in the section 'Connecting your Email to a Client App'
and 'Use the following type of encrypted connection' SSL/TLS
Once all set, OK out of that back to the previous screen and try the 'Test account settings' button.
There are general instructions here and a video (but you obviously have to use VM's own server settings as above)
https://support.microsoft.com/en-gb/office/change-or-update-email-account-settings-in-outlook-for-windows-560a9065-3c3a-4ec5-a24f-cdb9a8d622a2
If you still have an error, with all of the settings above correct, you may have an issue with the Outlook program (some general reports from earlier in the year from Outlook said they were finding the synchronisation was working intermittently)
Final suggestion would be that VM's email service exists in a state of perma-crisis. It is widely speculated on here that its future is uncertain (though VM has confirmed no plans to close it as yet). The best way to solve all VM email issues is to give up using it and migrate your accounts and services to a third party email provider which is reliable. That also has the advantage of freeing you from a tie to VM should you ever wish to change broadband provider.
Hope you can manage to get it working.