I've inserted the video rotation filter into a playing graph, and this is the output I see when I use 90 degrees, 270 degrees, or diagonally:

Has anyone got any suggestions as to what the problem may be?

If I use 180 degrees, vertical or horizontal, the video image shows fine - it is only 90, 270 or diagonally that cause the problem. Having said that, vertical doesn't actually seem to do anything (the video image does not change at all, but I will worry about that problem when I get this garbled image problem worked out).