Hi,
The 200 code means that your server did respond to the upload request, but it failed to save a file. The PHP script from the guide page outputs the PermissionDeniedError message in three situations: there was no fileName parameter in the upload request, there was no valid upload file in the request, or the extension in the fileName parameter was not valid.
I suspect that what happened to your participants is that their file was just too big. I suggest that you increase the maximum post and upload file sizes.
Unless those uploads were asynchronous uploads (eg. UploadRecordings("label", "noblock")) your participants must have been prompted with a message containing a link to download an archive of their recordings. You could try to reach out to them to see if they saved it and can send it to you
I’m sorry for the inconvenience
Jeremy