Cannot add a client to a VM Backup policy

Using Avamar through Networker 8.2.3.1.  I tried to search for an answer, but something weird is happening on the site where I can't scroll down the page, so apologies if this is a duplicate.

There is one particular VM that I simply cannot add to any VM policy.  I got to vsphere web client, connect to the VBA, select the backup tab, click edit, find and check the server in the list, click ok and get this:

Backup policy was edited successfully, but the system failed to add one or more clients to the policy.  Please check the event manager for specific details.

Looking at the Event Manager entry is spectacularly unhelpful, as it basically says "There was an error".

There's nothing obviously different about this VM.  It has a space in the name, but I have other clients with a space in the name.  It's not overly large (128GB).  It's VMVersion 8.  The only thing I can think of is that it was apparently part of a backup policy when it was on a different ESX host, as I've seen log entries referencing it, but it does not appear in any of my current policies.

Any ideas what's going on here?