Attached is the current log for the 2nd client which I already had in place, I've created log directories for the other client so we can check tonight.
I opened port 1556 as suggested on both clients and loaded host properties, and ran a full backup.
Host properties loaded with in a minute or two on both clients, which is a lot better than in the past.
Backups ran sucesfully for both drives on both clients. I had one client error out with status 13 on the system state, but when I reran the job it completed sucesfully. The other client skipped a file, but when I reran the job it ran without issue.
Given that these clients have been intermittent I'll need to watch them for a few days to see if the opening of port 1556 was a solution.
Typically our domain policy is set only to open ports 13720, 13724, and 13782 which allows all of our clients to backup, but given these clients are a bit atypical in setup I wouldn't be suprised if the opening of 1556 was what they needed.
I'll update by Friday afternoon on how the next few days go, and post further logs if we have more failures.