Connecting server to farm using Connect-SPConfigurationDatabase : Operation is not valid due to the current state of the object

I am trying to connect a server to a farm using the Connect-SPConfigurationDatabase Cmdlet:

PS C:\Users\sp_admin> Connect-SPConfigurationDatabase
cmdlet Connect-SPConfigurationDatabase at command pipeline position 1
Supply values for the following parameters:
DatabaseName: SharePoint_Config_7f6dbfc7-63ac-xxxx-xxxx-975c08dc44ed
DatabaseServer: SERVER-NAME\SHAREPOINT
Passphrase: ***********

I get this error:

Connect-SPConfigurationDatabase : Operation is not valid due to the current
state of the object.
At line:1 char:1
+ Connect-SPConfigurationDatabase
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidData: (Microsoft.Share...urationDatabase:
   SPCmdletConnectSPConfigurationDatabase) [Connect-SPConfigurationDatabase],
   InvalidOperationException
    + FullyQualifiedErrorId : Microsoft.SharePoint.PowerShell.SPCmdletConnectS
   PConfigurationDatabase

When I check the logs, I get a little more detail:

The following error was encountered while unjoining the farm: System.InvalidOperationException: Operation is not valid due to the current state of the object.     at Microsoft.SharePoint.Administration.SPConfigurationDatabase.Microsoft.SharePoint.Administration.ISPPersistedStoreProvider.GetParent(SPPersistedObject persistedObject)     at Microsoft.SharePoint.Administration.SPServer.Delete()     at Microsoft.SharePoint.Administration.SPFarm.Unjoin()    25b0e8e6-xxxx-xxxx-xxxx-6f6075f54bec

I'm not sure where to look next for details of this error or how I can fix it, would anybody please have any ideas?

Thanks.