Since last night's maintenance, every course image and PDF returns 404

The MasterLinux school serves the images, diagrams and PDFs of its courses from medias.masterlinux.lab, hosted on a single EC2 instance, ml-medias-01 (nginx). Trainers upload their files to /mnt/medias. A cron job copies this folder every night at 02:00 to the S3 bucket masterlinux-medias-sauvegarde.

Last night (Monday 19 October, 22:10), the operations team stopped and then started the instance to move it from c5d.large to c5d.xlarge (resize). The restart went fine, the instance shows "2/2 checks passed".

This morning, students report that course pages no longer display any diagram and that PDFs return 404. Only a few files uploaded by trainers since 07:00 are displayed. Today's sessions start at 09:30.

You are the on-call administrator, connected over SSH to ml-medias-01 (account ops, sudo, AWS CLI v2 with the instance role).