After upgrading vCenter from 6.0 to 6.5, Veeam was failing all jobs. I tried to rescan the vCenter servers, but that failed also. It took me awhile to figure out what was going on. It turns out that (at least for me) upgrading vCenter server to 6.5 does not give the "root" user access to vCenter. Who would'a thought, huh? I could actually login to vSphere web client with root, but it wouldn't show any of the vCenter servers, hosts, vm's, etc. Since I had always used "root" to connect to my vCenter servers from Veeam, this was my problem. I was able to give "root" permissions and now everything is fine. Of course I could have used another user or setup a new one to accomplish the same thing.