So when trying to read from excel files we got this message repeatedly, and could not figure out what the hell it meant. We tried reinstalling some dlls that were suggested by other blog posts, but nothing would make the excel file open. Turns out, our connection string was wrong.
This is a basic correct connection string.
"Provider=Microsoft.Jet.OleDb.4.0;data source=[FILE_PATH];Extended Properties=Excel 8.0;"
UPDATE:
This guy knew the answer if I had read my opened tabs.