I got the following error:

Exception returned from back end service. System.ServiceModel.FaultException`1[System.ServiceModel.ExceptionDetail]: AccessDeniedEx:The current user has insufficient permissions to perform this operation. (Fault Detail is equal to An ExceptionDetail, likely created by IncludeExceptionDetailInFaults=true, whose value is: System.UnauthorizedAccessException: AccessDeniedEx:The current user has insufficient permissions to perform this operation.
at Microsoft.SharePoint.Taxonomy.MetadataWebServiceApplication.CheckPermission(SPIisWebServiceApplicationRights requiredPermissions)
at Microsoft.SharePoint.Taxonomy.TaxonomyDatabaseMapper.GetTermSetWithAllTerms(Guid rawPartitionId, Guid termSetGuid, Boolean includeDeprecated, Boolean limitMembership)
at Microsoft.SharePoint.Taxonomy.MetadataWebServiceApplication.GetTermSetWithAllTerms(Guid rawPartitionId, Guid termSetGuid, Boolean includeDeprecated, Boolean limitMembership)
at SyncInvokeGetTermSetWithAllTerms(Object , Object[] , Object[] )
at System.ServiceModel.Dispatcher.SyncMethodInvoker.Invoke(Object instance, Object[] inputs, Object[]& outputs)
at System.ServiceModel.Dispatcher.DispatchOperationRuntime.InvokeBegin(MessageRpc& rpc)
at System.ServiceModel.Dispatche…).

Metadata backend service request ExpectedFailure: Metadata backend service request failed in a authorized access exception: System.ServiceModel.FaultException`1[System.ServiceModel.ExceptionDetail]: AccessDeniedEx:The current user has insufficient permissions to perform this operation. (Fault Detail is equal to An ExceptionDetail, likely created by IncludeExceptionDetailInFaults=true, whose value is: System.UnauthorizedAccessException: AccessDeniedEx:The current user has insufficient permissions to perform this operation.
at Microsoft.SharePoint.Taxonomy.MetadataWebServiceApplication.CheckPermission(SPIisWebServiceApplicationRights requiredPermissions)
at Microsoft.SharePoint.Taxonomy.TaxonomyDatabaseMapper.GetTermSetWithAllTerms(Guid rawPartitionId, Guid termSetGuid, Boolean includeDeprecated, Boolean limitMembership)
at Microsoft.SharePoint.Taxonomy.MetadataWebServiceApplication.GetTermSetWithAllTerms(Guid rawPartitionId, Guid termSetGuid, Boolean includeDeprecated, Boolean limitMembership)
at SyncInvokeGetTermSetWithAllTerms(Object , Object[] , Object[] )
at System.ServiceModel.Dispatcher.SyncMethodInvoker.Invoke(Object instance, Object[] inputs, Object[]& outputs)
at System.ServiceModel.Dispatcher.DispatchOperationRuntime.InvokeBegin(MessageRpc& rpc)
at System.ServiceModel.Dispatche…).

Solution

If you are running your Search Service Application using a different account than the farm account then you need to grant “Full Control” permission the this account on the Managed Metadata Service Application!