I have the same issue at two of my client and have yet to figure it out. I have it working in a similar fashion in my LAB but not for these two clients. I opened a case a long time ago and they said it was my firewall blocking communication but I have since build the environment all on the same network to rule out routing and firewalls, and get the same error. I even built a whole new Veeam server environment to test with but it has the same error again. to see if something changes in Veeam9 I upgraded and I get a different error. In Veeam 9 I get "Error: Source WAN Accelerator error: Reconnectable protocol device was closed."
I will be opening another ticket soon, but I thought I would post here to see if anyone has more information.