Video upload fails on mobile — fluent-player/video-upload returns 0

A member gets this error when trying to upload a video from
mobile (see screenshot). The video is 180MB (well under the
300MB limit), and the same file size uploads fine from other
devices.

Error: fail to post .../wp-json/fluent-community/v2/fluent-player/video-upload 0

What causes the "0" response here, and how can mobile uploads
be made more reliable?