Apologies for hijacking the thread but I am in a similar situation.
I recently started evaluating Veeam backup&replication.
We have 2 domains, no trust between them. Domain A & domain B, Veeam is on Domain A
I have used the credentials manager to create accounts for my 2 domains.
While everything works great for domain A, I cannot get the guest index/application-aware to work for domain B.
I assign different credentials before testing on the backup job for the servers in different domains.