Copilot in MS Word suddenly reports that every uploaded file is empty, corrupted, or in a format it cannot process

Question

Copilot in MS Word / Microsoft 365 suddenly reports that every uploaded file is empty, corrupted, or in a format it cannot process, including PDFs and MS Word files that previously worked. The files are under the upload limit and are supported formats. Please check whether there is an account, service, or upload-ingestion issue. All the same files were uploaded without problems previously. I can identify no system settings that were changed on my end.