So I got 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. I already tried a server reboot and an IIS reset, usually those worked like a charm. But not in this case. I also didn't want to rebuild the Managed Metadata Service.
I checked the WSS_ADMIN_WPG on my server just to see if my app pool account was part of this group. The account needs this group to be able to write on local files, directory, whatnot. After adding my app pool account to this group I ran a PowerShell cmdlet: Initialize-SPResourceSecurity After this my Managed Metadata Service worked again.