Unable to upload resource

A friend for whom I created a simple website is unable to upload PDF files despite having had no trouble for ages.

We're running Perch version 3.1.7, and PHP version 7.4.

I can't back up and upgrade to 3.2 because I can't figure out how to back up the website, including the database.

Anyway, the error message is a 403 forbidden - see screenshot.

Can anyone help?