I did find your post a little confusing, since you're using 'server' to refer to both the client systems and the system they are connecting to. I'm still not sure which one the ubuntu system is. I'd have used 'MachineA' and 'MachineB' and talk about them as the clients. ok, enough with the nits...

It's failing on the initial TLS negotiation. As it's obviously getting to the server, it's not a network or firewall issue. That leaves either certificates or TLS library version mismatch as the most likely possible culprits. If you're able to login to ServerB (as the user the task is running under), I'd try a local ftps client there to see if it can login to your ftps-server. If you're able to make changes on the ftps-server, try bumping up the debug level on the server app. Most servers have a diagnostic level that provides detailed info on every failed session. It will likely be more informative.

fnord