SharePoint 2013 – The tagging service is currently unavailable

Today I tried to post something on my News Feed using a hash tag, but was promptly hit with the following error message:

Something went wrong
The tagging service is currently unavailable.  Please try again later.

I was also unable to manage categories for wiki pages.  I began investigating by logging into Central Administration and navigating to the Service Application page.  I clicked on the Managed Metadata Service from the list and encountered this error:

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

The service was started, as were the application pools, so I reviewed the ULS logs on the server running the Managed Metadata service and this was being logged every few seconds:

Error encountered in background cache check System.Security.SecurityException: Requested registry access is not allowed.

After a bit of Googling I found many solutions, none of which worked for me.  Eventually I ran the Initialize-SPResourceSecurity cmdlet on the application server hosting the Managed metadata service and I was then able to access the Managed Metadata Service Application in Central Administration.  I then ran it on all other servers in the farm which fixed the issue with hash tags and wiki categories.

It may be a coincidence but this stopped working after I installed the March 2015 CU.