I’ve been testing and testing and finally I can say:

– All the problems in pricing backend page was due a javascript conflict with other plugin.
– The single upload feature is still not working, even if I create the folder manually via FTP.
– The upload in zip format works perfectly. The folder for the gallery is created if it doesn’t exist (so I think that the single upload problem it’s not related with permissions).
– The “Scan folder” feature works perfectly.