trumpy81 told me: limit the number of files in each folder to 200 only, if you need to, create numbered folders like:
2011 07 Crista - 01
2011 07 Crista - 02
Are there other limitations of this kind? At present only a few hundred of my pictures have gotten thumbnails and the whole Plex App is behaving in an unstable way.
In its present shape, It is really useless to me.