Got hit by this error today.
System.Data.OracleClient requires Oracle client software version 8.1.7 or greater.
I remember getting it earlier but forgot the cause and the solution, so I want to document it now.
Basically this happens because IIS user has no permissions to read & execute files in Oracle's directory. So the solution is to manually grant the permissions.
Details in this Roy Tore Gurskevik's post.
