I think I more or less narrowed down the problem!!!**
I just discovered that I couldn't even use the Crop effect anymore, even though I could when i just installed the program. So, I unloaded all of the VirtualDub plugins that I got from some huge pack, and also three separate ones*. And now Crop works again. I'll update when there's more progress (just in case someone within three years or so will find this thread, having the same problem. I hate it when I google a problem, and find a potentially helpful thread which then just seems to stop in the middle )
Btw I strongly suspect "Old Movie" msu_oldcinema_20.vdf to be the culprit. It doesn't really work too well anyway: most of its option won't stick (you can set them after going to some trouble, but they will snap to default anyway). Too bad, it has the huge advantage over the default "Old Film" in that can leave the coloring as it is. Anyway, I will no longer just load plugins without needing them, but I can ofcourse still use it in a dedicated mini-project, export the result and use that in a general project.
*(I work with Photoshop a lot, and whenever I have to reinstall, I'm in the habit of installing as much plugins as I can lay my hands on. Even the faulty ones only cause trouble when called upon, not if they're only just there. So, I assumed, without a conscious thought, that VideoPad would act the same. But apparently 'loading a plugin' does involve more than just indexing the path towards the plugin location.)
