Just as a test, fill the gap with a blank clip. Also, before exporting, allow the green caching line that's above the thumbnails to complete.
In the export menu, click the Default Quality /Filesize blue link and switch the compressor to MPEG. Test other file formats as well, such as AVI.
This may be a Mac-related issue. Mention that and include the link to your project when you relay your findings to NCH. Click the chevron at the right-top of the VP window for a link to a bug form.
You might also try this older version to see if it succeeds at 1080. Retain your installer and licensing info, if any.