When trying to connect to an Oracle database you receive the following error:
"OCIEnvCreate failed with return code -1 but error message text was not available"
The reason is that you have multiple versions of the Oracle Client installed on your local computer which are causing a conflict.
The solutions:
1) Install a supported version of the Oracle Client.
2) Perform a search for "OCI.DLL" on the local machine. If you find any instances of this file in any location other than your ORACLE_HOME rename or delete it. If in doubt, uninstall the Oracle Client, then perform the search again. Anything that is leftover is the culprit.