Thanks it was a render issue. Although the clips played fine, in trim mode they vanished. Rendering fixed that…. until I started trimming with the JKL keys, and then I lost the picture again. The solution was to turn on rendering on-the-fly as suggested. the picture disappears during the JKL trim operation, but returns immediately trimming pauses, without having to leave trim mode and re-render. Even better feedback is obtained using the M<>? keys as the picture is not lost at all, even momintarily (although there is no sound this way).
So thanks for this great tip. I was wondering if another solution would be to temporarily disable the effects – but I could find no way (or mention) of this being possible, so I assume not. A global effect disable would also be great.
Many thanks, Ken.