Sorry, already found the cause:

I'm developing and testing on a computer running Vista so I needed to deploy for the IIS7 integrated mode. I originally added that line to the wrong section of the web.config file.

 

My webapplication will eventually be installed on all kinds of Windows servers, so I won't know in advance which deplyment method to use for the UploadModule entry.

Can I simply add that line to both sections of the web.config to accomodate all IIS servers? Or could that cause other problems?