I have installed, upgraded and activated! After your post I actually uninstalled and did everything again, just to be extra sure. It still doesn't work.
I am at a total loss, especially since now the link between browse list and individual items seems to be lost in browse collections (goes to the entire browse list instead), but not in CollectionTree...
Is there any code you may need to see what may be causing the problem?
As an example: http://arpc65.arm.ac.uk/omeka/collections/show/14 is a small sub collection, which can be found via CollectionTree. To test ItemOrder I moved ARM WFAE 3.3P up to the top. It does not appear there however. When I open the ItemOrder admin page again, the order has been put back to the original, even though I do not click that particular button.
Thank you for your help.