Do you see anything strange in the Console tab of the Developer Tools (F12)?
Note that we do not support Internet Explorer 9 anymore in Mendix 6. See system requirements: https://world.mendix.com/display/refguide6/System+Requirements
Not even Microsoft supports it anymore: https://www.microsoft.com/en-us/WindowsForBusiness/End-of-IE-support
Thanks for your quick response.
Is there I way to alert the user that they have to use IE 10 + ? My customer uses IE9 so I show the message 'Please upgrade'