It's just a guess, but maybe it helps to delete the cache. Start videopad, do NOT load anything (or press CTRL + N = New project). Then go to Menu -> Tools -> Options -> Disk. Click the the button "Clear Unused Cache Files". When videopad has cleared the cache, load your project/videos.
If it doesn't work: Maybe it is just one if the video clips, that leads to the problem. Try to pinpoint it, load one video after another. Maybe one of your clips is broken?