Hi Elisa,
Interesting, we haven't seen that before. With a blank MySQL error message like that there may not be a lot we can do since there's nothing to work off of to identify the problem. However, if you can send your logs to support@neversettle.it we can take a look and see if those provide any clues. It's a little verbose, but here are some detailed steps on what we need:
http://support.neversettle.it/knowledgebase/articles/462399-how-do-i-request-support
(If you're running a local or non-public dev site where those log URL's wouldn't be accessible to us you can just go into wp-content/plugins/ns-cloner-site-copier/logs and grab those actual files and attach them to your email.)