Sometimes in a SharePoint 2013 farm we recognized that the managed metadata service or connection is currently not available as error message. In the event logs it shows the id 8088.
After googling it says we should check the permission of the service user, the app pool is running, we performed an iss reset, we restarted the service but nothing of this helped.
The solution which solved our problem was to go the Internet Information Service (IIS) and click on the advanced settings of the application pool which runs the managed metadata service and re-add the same service application pool account for this.
After recycling the app pool everything runs fine again. Maybe it helps you, too.