Looks like in the logs the process was in the final steps and when trying to package the video together and wrap it up, it lost track of the temp file that was created and errored out.
This can happen if the system goes to sleep, loses the file path, or hits an error within the final steps.
Did the system go to sleep?
I would recommend moving the original source file from the downloads folder and into a clean folder option and then bring that into the app for processing. In some instances I have seen the downloads folder get cleared or a file moved after a process was started and that cause an error. Usually this is when the file is in a cloud-synced folder though.
Glad to hear that it worked on the second attempt. I will share the logs with the team for further reivew on what happened the first time then to see if they can sort it out.