I made a change so that “blank” and “<tractor>” do not appear as missing files or block export. That does not fix the root cause of how these are introduced, and it does mean the root issue will be masked further in the future. I have heard people report about this in missing files or the related “INVALID” in video output for years here and on social media. “INVALID” may appear due to an invalid <tractor> but also for other reasons. It seems for the “blank” issue I have manually fixed for people that it often does not show as something ugly and unexpected in the output because blank is, well, blank.
The recent uptick in these incidents is mainly because version 21.01 added a check at export that is blocking:
Fixed Export File does not check for missing files.
Meanwhile, its appearance in Missing Files has been easily ignored. We added an Ignore button to this export error dialog for the next version.