Generally, I expect this to be either a busy server or a weak signal for the student's wifi. For almost all problems I suggest to my students that they make a point of "plugging in" rather than working off wifi, using a computer rather than a mobile device, clearing their browser's cached pages, and to try again in an hour or so (rather than the next day). If all that fails, I ask my students to contact canvas support and work with them directly to resolve the issue. (I also ask that they email a copy of what they were trying to submit, so that I can confirm they finished on-time.)
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