.net - Opening XLSX File Via OleDb Works In WinForms But Not Always Via ASP.Net?

I've built a simple library to import data from XLS/XLSX files. The code runs perfectly in my WinForms app but occasionally throws exceptions when I run it from ASP.Net. The "occasional" part seems to based on file size. My test file is about 16,000 rows and 18 columns totaling about 4MB as an XLSX file. If I drop rows (down to about 12,000) it works or if I drop columns (down to about 12) it works or certain combinations of these (14,000x14, etc) it works. This leads me to believe that I'm possibly running into a memory constraint somewhere. Below is the distilled code that's not working:

Dim dsn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=""c: est.xlsx"";Extended Properties=Excel 12.0"
Using Con As New System.Data.OleDb.OleDbConnection(dsn)
Con.Open()'Exception thrown here
Con.Close()
End Using

The exception that I'm getting is:External table is not in the expected format Searches for this all talk about getting the DSN correct and since it works on the desktop I know I've got that right.My test machine is a Windows 7 64-bit with the Microsoft Access Database Engine 2010 Redistributable - 32 bit installed. (My machine has Office 2010 32 bit installed which is why I have the 32-bit ADE installed, it won't let you install 64-bit.) I've set my IIS app pool to allow 32-bit applications which I needed to get ADE to work in the first place. I'm running the v4 Framework and as far as I know my IIS settings are pretty much standard.Like I said, the code works every time when I run it through a WinForms app but occasionally fails through ASP.net on the same machine.