HELP! Hot add not supported, Case #01727476

[Gasp100]

Some work was done (is being done) on our VCenter server to try and get it upgraded from 5.5 to 6.0. For some reason this server upgrade process has been a nightmare (not my project) and I don't really know the status of the VCenter version right now but something has changed. ALL of the VEEAM backups that are handled by this VCenter are throwing this message and the transfer rate is dropping down incredibly slow and unacceptable.
"Hot add is not supported, failing over to network mode"
I also see a message "using guest interaction proxy ... different subnet" but this is NOT the case. The guest in question AND the VEEAM B&R server AND the specified VEEAM proxy are all on the same subnet and all machines are in the same VMware cluster and can see the same luns / datastores.

My main concern is that our Exchange server which has about 3TB of used data across a number of datastores was only 60% complete after 40 hours!!!
Now the transaction logs are filling up and if they completely fill up we are screwed.
Any ideas?
I'm thinking they messed around with VCenter versions and something is now FUBAR from VEEAM perspective.
Something similar to this below but how can I tell?
Hotadd may fail if any disk was created with a newer hardware version than the VM being backed up. For example, if a disk was moved from a hardware version 8 VM to a hardware version 7 VM. To resolve, upgrade the hardware version of the VM.

[DaveWatkins]

Have you re-scanned the vsphere server in Veeam since it's been upgraded (I assume the vsphere server has been upgraded already)?

No idea if it will help but I doubt Veeam likes having it's vsphere host change version without notification

[foggy]

I'd check the hotadd proxy that was used prior the vCenter upgrade, whether it is still available, has access to the datastores, and all other limitations that might apply as a result of the maintenance process.

Please also note, that in general guest interaction proxy and transport proxy are different components (though both can be run by the same server).