server error

[ravikumar]

It is invalid to show a modal dialog or form when the application is not running in UserInteractive mode. Specify the ServiceNotification or DefaultDesktopOnly style to display a notification from a service application

what is could be

 

[vis781]

You can't display a VB MsgBox in an ASP.NET application. The ASP.NET code
runs server-side, but you want your message box to show on the client.

Simply use the client-side Javascript alert() function, depending on what it
is you want to achieve with displaying the message box.

Also there is a really good article here, unfortunately in spanish, written by a MVP about how to use a message box in ASP.Net

http://www.microsoft.com/spanish/msdn/comunidad/mtj.net/voices/art120.asp