When accessing the Managed Metadata Service Application from a Content Type Syndication Hub, the following error might occur when opening the Term Store Management Tool:
The error does not occur, when the Term Store Management Tool is opened from the central administration site.
The ULS log on the WFE server logs the following message:
Failed to get term store for proxy ‘Managed Metadata Service Application Proxy’. Exception: System.Security.SecurityException: Requested registry access is not allowed.
The MSDN blogs titled “SharePoint Error: The Managed Metadata Service or Connection is currently not available. The Application Pool or Managed Metadata Web Service may not have been started.” and “ The Managed Metadata Service or Connection is currently not available” discuss this issue also. Although this pushed me into the right direction, it did not solve the issue in my case.
Further investigations on this issue resulted in the finding, that the registry key HKLM\Software\Microsoft\office Server\15.0 did not have the right permissions. In fact, the permissions for the “WSS_Admin_WPG” and “WSS_WPG” were completely missing. This in contrast to the TecNet article titled “Account permissions and security settings in SharePoint 2013”, which states these permissions are needed.
Executing the following PowerShell command helped to solve the issue, adding all permissions needed: