Strange. What Videopad version? Clear Unused Cache Files, under OPTIONS | DISK tab.
If no go, save your project, then install the latest. For licensed users upgrades are free for up to six months from purchase date. After that, VP will continue to fully function but a fee will be required in order to register the newest. Retain your old install file and registration info.
Still no joy? Please upload and share a file that does this. It's easy, takes only seconds.
Upload the MTS to Dropbox, Google Drive, Microsoft OneDrive, etc. Copy (share) the URL and paste it into your post here.