We've had this problem as well, and the following usually fixes it:
1. Visit the user's account settings in Canvas. Unregister Google Drive under "Registered Services."
2. Under "Approved Integrations," delete Canvas.
3. Within the student's Google Drive account, visit "Settings" and then "Manage Apps."
4. Next to Canvas, click "Options" and "Disconnect from Drive."
5. Students will now have to re-authorize Google Drive from the LTI assignment page, or re-register Drive from the user's settings page in Canvas.
As mentioned by others, personal Google accounts also cause a problem. I most often see this issue when students are using Canvas at home--if they have their personal account open/active in any other tab or window within the browser, it'll make Canvas grumpy. For students/families who are heavy Gmail/Drive users at home, I recommend using a different browser for Canvas to keep everything separated.
These two fixes have solved the problem in about 95% of cases!
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