I don't have any details about the comic generating the corrupted jpeg so I can't check further unfortunately.

Well too bad I can't share my purchase in my own family.. that's disapointing.

I checked a little bit further and nailed down the root cause of the sync not working properly. It was not because of the nolock option (because I was already on nfsv4, so there no such option available) but rather because I was behind a Cloudflare reverse proxy. Can you confirm the sync feature is using tcp instead of http ? anyway, connecting directly to the server inside a Cloudflared tunnel does work but outside the tunnel and using a regular http proxy, the sync fails. maybe one day, I'll find a solution but for now, it will stay like this.

It uses http.

And yeah, I am not against Family Share, but I don't know why I don't have any option to enable it in YACReader, and I am not the only one, I'll try to contact Apple about it.