For security reasons DTD is prohibited in this XML document
Identity Client Runtime Library (IDCRL) could not look up the realm information for federated sign-in
While the issue is out of Veeam’s support scope, what follows is a list of solutions customers have reported resolved their issue:
Some customers have reported that they were able to contact their ISP and request that the DNS relay or DNS helper feature be disabled.
Create entries within the hosts fileC:\Windows\System32\drivers\etc\hosts of the Veeam Backup for Microsoft 365 server to force the msoid domains to resolve to the loopback IP of 127.0.0.1.
Add a CNAME record on your DNS server as shown here:
This form is only for KB Feedback/Suggestions, if you need help with the software open a support case