That error suggests you have your Jellyfin server exposed directly to the internet. That is not a good idea.
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Is it okay to expose it behind a reverse proxy?
I personally wouldn’t do that either but plenty of people do. It depends on your tolerance for risk.
I use a VPN tunnel via an external endpoint. It’s more work but means I don’t have any devices directly accessible from my home IP.
Two suggestions and apologies if you already did these but they are a common problem people can have:
1 did you set the library as a movie library or maybe accidentally something else? Because if its e.g. set to shows it may not find the Movie metadata.
2 in case you use linux, but seeing your other comment that might not be the case, make sure the folders your movies are in have the right permissions for jellyfin to access them.
Permissions seamed to be the issue. Everything is working, all be it, a bit slower then plex used to. But least it's working.
Jellyfin expects movies to be in sub-folders with their movie titles for the best experience. You can have a Star Trek library, but each movie inside of there needs to be in its own folder. For example M:\Star Trek\Star Trek II - Wrath of Kahn\Star Trek II - Wrath of Kahn.mkv
This has always worked the best for me.
So, just to give a different perspective. I don't do that, all movies are in one folder, and it works still. Might be worth to try of course.