You reached a .NET 2.0 defect. If a folder is deleted, the session variables are lost.
The xlsx file is a zip file. EasyXLS use a temporary folder to unzip the files. After the load of the file, the folder is deleted.
You have 3 alternatives:
1. Use .NET 1.1, instead of 2.0
2. Use an xls file instead of an xlsx file (if possible)
3. Change the session mode to SQLServer, instead of InProc
The following link might help you:
http://forums.asp.net/p/998370/1311015.aspx