When I try to insert a record, I get this error : The underlying provider failed on Open.
This error occurs only with IIS and not with VWD 2008\'s webserver.
In the EventViewer
In my case, I was using Sql CE, and my connection string was set up with an absolute path instead inf the |DataDirectory| variable. Changed this and it began working on the server.
Obviously, it worked fine on the development machine.