There was an unexpected error loading this library

Every time that I access plex in synology NAS 918+ and I access my media movie library the following message shows up

There was an unexpected error loading this library

After a while and when I open the movie library on another tab its gone. Why?

Another doubt, if the movie is not found in plex database why can’t I add to my list?

The network settings are correct, I use DHCP, so there is no network configuration needed. My Internet access is always on and working, the DNS that I use, have a quick response time. I don’t see any problem with the Internet connection.

Where can I find the metadata? It is saved in the NAS? Where can I find it?

I’ve downloaded the Plex file, from the plex website, there was no difference.