Ok, that's makes sense. Probably the main exlibris library is not able to read the spine from the EPUB for some reason.
I got reports from fellow Italian users, using Windows/Linux machines, and almost always the plugin was working properly. However, in one case the plugin was not behaving correctly, and it was a portable version of Calibre.
I suspect that it is due to the exlibris attempt to create a temporary directory for unpacking the EPUB and examining its content, but I have to better explore the issue.
(I assume you upgraded correctly the plugin to the new version 1.0.10 --- please check the 'About' dialog out --- and restarted Calibre, right?)