Content Index State: Unknown Single server no DAG

Hello Everyone,

I have a problem which seems to be quite unique, (or maybe not)

My setup is as follows, single Exchange 2013 box, (OS Server 2012 Standard)

1 Mailbox DB (MB DB1)
1 Archive DB (MB AR1)

When I run Get-MailboxDatabaseCopyStatus the Archive mailbox (MB AR1) has a Content Index State of Unknown, but the MB DB1 is listed as Healthy.

Also, if I do a search of items in the Archive MB from Outlook or OWA it returns nothing. Has anyone ever experienced this?

Kind regards,

Anthony