Symptom
When attempting to connect to a SQLServer SDE Geodatabase using an SDE connection file we receive this error when testing or trying to connect to the database:
“Could not open the Enterprise Geodatabase. The error number from ArcObjects is: '-2147216072'. The error message from ArcObjects is: {Underlying DBMS error[Unknown error. No extended error.]}.”
Resolution
This may not be the only reason why this error occurs, but it does happen without fail when the ODBC Driver for SQL Server isn’t installed.
This can be remedied by downloading the ODBC Driver from the Microsoft website. Once it is present FME will be able to connect to the Geodatabase.
More information on finding ArcObjects error numbers can be found in FME and Esri ArcGIS Troubleshooting Guide.