Have you tried CSE?
Cic clarifies on their login page as below. Save a screenshot of this page to cover your bases and keep on attempting to send through webform. I am not as experienced and expert as you and several others on this forum, but just trying to help bit I can by giving my 2 cents

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- If you are unable to submit a document on time as a result of the technical difficulties, try again over the next few days. If you are concerned about missing your deadline, submit the requested documents using the IRCC Webform.
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