Check the DNN error logger. Does it log any kind of errors when you try to use the Image Manager? It could also be a permissions problem. Make sure your normal user has the correct permissions to any folders you want them to traverse through the DNN File Manager.

That's weird Geert. If all the files and database are the same between your dev and production server, the only thing I can think of is maybe NTFS permissions are causing some problem. I believe I had to do Ctrl-F5 refreshes after making changes to the config or permissions for things to take effect for RadEditor. So keep that in mind in case you are not already doing it. One last thing you could try is to use the RadEditor provider on CodePlex. http://radeditor.codeplex.com/ For me, neither worked until I logged in as a non-superuser/host account, and then both worked. But it seems you are having a different issue, so maybe the different provider will make the difference. It installs just like a module, and has pretty decent documentation on codeplex.