The full error message is
“Showing a modal dialog box or form when the application is not running in UserInteractive mode is not a valid operation. Specify the ServiceNotification or DefaultDesktopOnly style to display a notification from a service application.”
This is probably due to you using MsgBox(“so message here.”) for your web application. I learn that you can only use MsgBox for window form application.
So, the solution is to think of other way to display a message to the user.