Getting this error on a SharePoint 2013 farm when trying to access term store:
Failed to get term store for proxy 'Managed Metadata Service'. Exception: System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
Looks like the most common way to solve this is to recreate your managed metadata.
I did a little investigation. And nothing worked.
At the end i just recreated it...
Be sure to have a backup.
Detach The metadata DB
Delete and then recreate the managed metadata service application.
Attach the old DB and in the metadata settings point to it.