thanks for the quick reply. I think i found the problem. I selected "Entire Library", that seems not to work. When i change to "Local node" it works fine. However, is still cannot create a filter for the video node, there is no entry in the right-click pop-up.
Another, smaller problem: when i add a new tag value to a video file (in a custom tag field of type "enumerated"), that value will not appear in the selection list of that tag field, when i add it to the the same tag field in a music file, it will appear in the selection list of both file types.