/lambλda/calculus/

This generic error message is often an indication of a syntax error in the connection string. In my case the error occurred when I tried to add a second Extended Property, like so:

var connection = new System.Data.OleDb.OleDbConnection(
    "Provider=Microsoft.Jet.OLEDB.4.0;" +
    "Data Source=" + path + ";" +
    "Extended Properties=Excel 8.0;HDR=No;");

The problem turned out to be that the multiple extended properties were not quoted. A single extended property does not need quoting, but multiple do.

The fix was then simply to add escaped quotes:

var connection = new System.Data.OleDb.OleDbConnection(
    "Provider=Microsoft.Jet.OLEDB.4.0;" +
    "Data Source=" + path + ";" +
    "Extended Properties=\\"Excel 8.0;HDR=No;\\"");