I was getting this error:
Admin OM Exception occurred. Message: Failed to communicate with the WCF service. , Error Code:e107, Stacktrace:Microsoft.SharePoint.Search.Extended.Administration.Common.AdminException: Failed to communicate with the WCF service. —> System.ServiceModel.FaultException`1[System.ServiceModel.ExceptionDetail]: The trust relationship between the primary domain and the trusted domain failed.
This was a great blog to point me to the issue. Turns out I had two domains in my Active Directory that had lost the trust setup but nothing was broken with my Active Directory or FAST setup. I had to delete the trusts and add them back, then it started working again.