Hello everyone,
Finally, yesterday evening I was able to resolve the issue. For some reason I am not aware of, the report template was not correct on my second step of the flow (weird because I know very well that I selected the correct one). Basically, the issue was with step 2 of my flow, not the upload to OneDrive, which was failing because there was no content coming from the survey123 report.
Thank you very much for your suggestions. I learned my lesson from this experience.