I had this issue this morning when trying to change the playspeed of a video clip. I changed it to play at 50% speed, then previewed it once. After that, preview doesn't work any more. The pause button gets stuck so you can't click play. Also, dragging the scrubber over the clip doesn't even flash through the video in the preview windows. I closed out Videopad to reload it, but it wouldn't run - clicking on desktop icon just doesn't do anything. I had to log off and log back into Windows for VP to run. Then it works fine until I apply the video speed change, and then it breaks it again every time.
What I ended up doing was just exporting the movie clip immediately after changing the speed, even though I couldn't preview it first. That seemed to work fine, as the exported video clip came out the way I had intended.
I have another sequence later on in this project that I had planned to change the video speed. I guess if nothing else I can make the changes and export, but just not be able to preview to verify first.
Is this a known issue or does anyone have any ideas?