Are you perhaps connected to more than one vSphere server?
What is in $global:defaultVIServers?
When I connected to the old vcenter server I thought I was connected to only one. Turned out I was connected to two. I removed one and it's running now.
This explains why in the new set of vCenter servers this is happening because the vcenter servers are in linked mode.
How can I get this to work with linked mode vcenter servers other than connecting individually?
Did you try by using the Server parameter?
You could do for exampleGet-InventoryPlus -Server $global:DefaultVIServers