era nelle FAQ del sito produttore, riporto il testo per chi ha lo stesso problema:
Thumbnails are created "on the go" using showthumb.php script. Please make sure that /plugins/system/videobox/showthumb.php file on your server is accessible from client-side. The easiest way to do that is to right-click on thumbnail (in that case no image icon) and select 'Open image URL' to see what error you get.
Most common reasons for errors at this point are:
- If you get error Warning: getimagesize(): http:// wrapper is disabled in the server configuration... when you open thumbnail image URL, then your php configuration is probably blocking access to external data. For thumbnail creation, Videobox must be able to open images from YouTube and Vimeo servers, otherwise it won't work. In most cases, you should set alow_url_fopen=1 in your php configuration. If you for any reason won't do that, you can still add thumbnails for these videos to /plugins/system/videobox/thumbs folder and give them video ID as filename (/plugins/system/videobox/thumbs/xrLx3YJkJqg.jpg for YouTube video with id xrLx3YJkJqg).
- file /plugins/system/videobox/showthumb.php has wrong permissions (chmod); most servers throw error 500 when you're trying to access file with permissions 766 or 777. Make sure they're set to 755 or 744 (in some cases you want them to be 644 or 655).
- scripts in /plugins/system/videobox/ folder are not allowed to be run directly; sometimes servers are set to prevent direct execution of scripts in folders other than site root folder. This results in error 500 (or sometimes 403). Check .htaccess file and server settings to make sure you can run scripts located /plugins/system/videobox/ folder.
- folder /plugins/system/videobox/ is protected and can't be accessed; in that case, you're most likely to get error 403 or 500 when you open showthumb.php file and also you may have some troubles with videobox style. Check .htaccess and server settings to assure this folder can be accessed from client-side.