nostr:npub19cvahxwvggsl00neu2fc2gjtut2uwm0mp6quyyw4p87s33z4xdtqfmqnc3 I found the underlying issue, describe in this ticket:
https://github.com/mastodon/mastodon/issues/22335

Since we use Scaleway, it was exactly our problem. I've now increased the 

S3_MULTIPART_THRESHOLD=5242880000

and now it works 🙂 

Thanks, Olav, for your bug report!