Hi!
Sorry for delay.
This error is mostly depend on connectivity issues. When we receive information from Manager and not correct information from host. The information of file to transfer is first which we check.
It could be affected by SSL chain. When we could communicate by SSL to Manager but not for all hosts.
You can go to setup of Virtual provides, and clear all certs, and switch on "Trust all Certificates" on Manager and each of hosts. Then run Sync and try to backup.
Regards