RelatedContent tabel remains empty, no matter what view I select

I keep getting the message: "No nodes are selected or a view does not have nodes as result directly such as the default 'archive' view", even if I select views of which I am 100% sure that they do return a result.
This must be something I am doing wrong, but I just cannot find what that might be...
Any ideas of suggestions would be greatly appreciated! Thanks in advance!