After days of tests done with the library I came to a conclusion, jriver loses memory / synchronization of the data loaded in the library.
I try to explain, my albums and images, as written above, are on the local pc, while my films on the remote pc in shared folders on the network. Since I find the server / client configuration inconvenient because every time you want to see a movie you have to disconnect the default library (where the TV is configured, among other things) to access the server library, I preferred to set all my local and remote folders in the default library.
Everything seems to work fine, the library is populated with cover art images and movie metadata, if I turn off the server pc the library shows a small prohibition icon in the cover art thumbnails of the movies, so this makes me think that the remote file data has been loaded in the library and that are synchronized with the remote folders, in fact the prohibition logo is saying that the files are not accessible.
When I turn off all the PCs and then turn them back on in the library I no longer find the remote files and jriver takes a long time to reload them.
Thanks