Roeland, Not sure what version of IE you are using, but in IE 8, the following should resolve this issue: - Go to the Tools menu and open Internet Options - Navigate to the Privacy tab - In the Pop-up Blocker section, click the Settings button - In the Pop-up Blocker Settings window, add the URL for your Mendix application to the list of allowed sites
That solved it for my application.
Good luck,
Mike
As Mike pointed out, this is a security setting. I've had similar problems at a customer, where users could not edit IE's settings. We've taken this issue to the IT department of the customer, and they added the Mendix application to a white list.