Access database is already opened. You better to close access DB and try it again.
If It does not work, let me know how did you share your access db to other users?

You are getting timeout error means, Set your timeout = 0 in sqlconnection and try it. If it does not work, insert 5,000 records at a time and put it in loop. It works.

Session("Cart") is not initialized yet. So make sure You need to check Session("Cart") with null(nothing) and assign it properly. You need to do something like this.

Dim cart As New Cart
cart =...