When I did a test of migrating our SharePoint 2010 intranet site to SharePoint 2013 I encountered a problem with the Managed Metadata Service. I am using SharePoint Server 2013 with December 2013 cummulative updates applied. After following steps at Microsoft TechNet of howto migrate the Managed Metadata Service application I received the following errors when I tried to open the term store management tool from central administration:

The Managed Metadata Service connection is currently not available. The Application Pool or Managed Metadata Web Service may not have been started. Please Contact your Administrator.

And in the ULS logs:
Failed to get term store for proxy ‘Managed Metadata Service Application Proxy’. Exception: System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.     at Microsoft.SharePoint.Taxonomy.Internal.XmlDataReader.GetDateTime(String name)     at Microsoft.SharePoint.Taxonomy.Internal.SharedTermStore.Initialize(IDataReader dataReader, Guid termStoreIdValue, Boolean fromPersistedData)     at Microsoft.SharePoint.Taxonomy.Internal.SharedTermStore..ctor(IDataReader dataReader, Guid termStoreId, Boolean fromPersistedData)     at Microsoft.SharePoint.Taxonomy.Internal.DataAccessManager.GetTermStoreData(MetadataWebServiceApplicationProxy sharedServiceProxy, Boolean& partitionCreated) b195739c-8011-5023-8da3-f7cd968a5d80

The steps I did to get the error above:

• Backup and restore Managed Metadata Service database in my sharePoint 2010 environment using SQL Server Management Studio
• Restore Managed Metadata Service database in the new SharePoint 2013 environment using SQL Server Management Studio
• Used powershell to create new Managed Metadata Service application: (where -DatebaseName ‘Managed Metadata Service’ is my restored database from SharePoint 2010 env.)

  New-SPServiceApplicationPool -Name 'ManagedMetadataServiceApplicationWebAppPool' -Account 'SP2013Services'
  $applicationPool = Get-SPServiceApplicationPool -Identity 'ManagedMetadataServiceApplicationWebAppPool'
  $mms = New-SPMetadataServiceApplication -Name 'Temp Managed Metadata Service Application' -ApplicationPool $applicationPool -DatabaseName 'Managed Metadata Service'
  New-SPMetadataServiceApplicationProxy -Name 'Managed Metadata Service Application Proxy' -ServiceApplication $mms -DefaultProxyGroup

• Open Central Administration -> Application Management -> Manage Service Applications -> Manage Metadata Service Application -> Error!

The solution

• Make sure you have started Managed Metadata Web Service in Central Administration -> System Settings -> Manage services on server
• Make sure the account you used in the script above and that the farm account have appropiate rights on database intended for migration (default db owner)
• Detach database in SQL Server Management Studio
• Using Central Administration, remove the Managed Metadata Service Application (and associated data – do not forget to detach database before or it will be removed!)
• Re-create Manage Metadata Service Application with a new database:

  $applicationPool = Get-SPServiceApplicationPool -Identity 'ManagedMetadataServiceApplicationWebAppPool'
  $mms = New-SPMetadataServiceApplication -Name 'Temp Managed Metadata Service Application' -ApplicationPool $applicationPool -DatabaseName 'New Managed Metadata Service'
  New-SPMetadataServiceApplicationProxy -Name 'Managed Metadata Service Application Proxy' -ServiceApplication $mms -DefaultProxyGroup

• Verfiy that the term store management tool page is now working
• Change database of the Managed Metadata Service application to your database intended for migration:

  Set-SPMetadataServiceApplication -Identity $mms -DatabaseName 'Managed Metadata Service'

• Run iisreset
• Verfiy that the term store management tool page is now working and that you can see terms from your old environment

I am not sure why this problem occurs. Possible a bug in SharePoint 2013? Please fill in if you have more information.