Unable to Move Mailbox to new Database – 0 of 0 Succeeded

I'm selecting one mailbox to move to a new mailbox database in Exchange 2013, and when I start the Migration job in ECP it shows one Mailbox, but a few moments later when refreshing the job it changes to 0 mailboxes and then completes in an apparent normal state with 0 of 0 succeeded. I deleted this job and all previous ones from the list and tried again – same result. I deleted the job again and moved on to investigate.

Digging deeper in the console I ran Get-MoveRequest and received the following:

[PS] C:\Windows\system32>get-moverequest

DisplayName                                    Status                    TargetDatabase
-----------                                    ------                    --------------
SystemMailbox{8cc370d3-822a-4ab8-a926-bb94b... Completed
WARNING: The object support.local/Users/SystemMailbox{8cc370d3-822a-4ab8-a926-bb94bd0641a9} has been corrupted, and
it's in an inconsistent state. The following validation errors happened:
WARNING: Database is mandatory on UserMailbox.

The mentioned SystemMailbox appears to be pretty old and was "Disabled" in AD. I tried enabling and disabling the account again, but no joy. I am not able to remove the request as I cannot identify it, nor have I had any results from repairing the source and destination folder of my migration using:

[PS] C:\Windows\system32>New-MailboxRepairRequest -Database CleanDB -CorruptionType ProvisionedFolder,SearchFolder,Aggre
gateCounts,Folderview

I am at a hard stop as it would seem the errors above are preventing my migration job from completeing and I have not been able to over come what I have found so far. Any advice? Thanks in advance.