Hi Hilda,
Welcome to Microsoft Community!
Per your description, it seems that when creating an external tool assignment from OneDrive, the students get the error with canvas assignment and unable to complete the assignment.
Considering your case, to narrow down whether the issue is related to Canvas or other factors, we would like to suggest you upload the same problematic Word document to OneDrive and try open it in the web to see the result.
In addition, if convenient, you can also check this similar one: Something went wrong, check your connection and try again
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Besides, may I know all files or just this specific file gets the issue?
Appreciate your effort to share any information with us. 😊
Best regards,
Gloria