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.
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.