It seems that Attachments and the qTranslate plugin cannot work together.
If I try to attach something (with the qTranslate plugin activated) the page content disappears, and the attachment's caption shows instead.
This happens only with the qTranslate plugin activated. (and of course I'll report the problem also to qTranslate plugin's page)
I've tried it also in a new installation with only the default wordpress (with 2012 theme installed) and the two plugins activated.