For some unknown reason, for some large files, V2C requests a Server-side upload (https://commons.wikimedia.org/wiki/Help:Server-side_upload SSU). However I have been told that this service is deprecated. So V2C should never request a SSU.
FYI, last October 2024, I processed several films and requested a SSU (https://phabricator.wikimedia.org/T378451). Now the files are not available anymore, so all this work is lost. :(((