Hi @NemanjaJanicic,
If this is a component server (and not a sub-role of your VBR server) than you can go to “Backup Infrastructure” and see the “Managed Servers” section. Here’s a screenshot that fits - even when it’s for v13, same for v12s:
https://helpcenter.veeam.com/docs/vbr/userguide/edit_server.html?ver=13
There you should see the server you mentioned and rightclick on it, select properties and click through the menu - you will see the credentials.
Maybe just a simple trick: Did you reboot the server before the upgrade? I’d always recommend rebooting before doing upgrades, doesn’t hurt and may solve some issues.
Maybe just reboot it now and see if that helps. If not and if you verified the correct permissions you should consider opening a Veeam Support ticket.
Best regards
Lukas