Recently we were looking into an issue with a SharePoint farm.
The event log was being flooder with “The Managed Metadata Service Proxy is inaccessible”, this resulted in us not being able to use or even browse the TermStore.
We headed straight in the ULS Logs and found the following error “System.Security.Cryptography.CryptographicException: Access denied”
After looking into the issue we discovered that the permission on the node in questions were incorrect on the C:\ProgramData\Microsoft\Crypto\RSA\MachineKeys directory.
We reset the permissions of the folder back to default, as found here https://support.microsoft.com/en-gb/help/278381/default-permissions-for-the-machinekeys-folders
We did the change on all SharePoint Servers on the farm and restarted them. After the restart the Metadata service came up with no issues!