The importer stopped working.

I have an importer working fine until a few days ago, now it is not working, it should be fired every 12hs via cron. It doesn't, no error, nothing, it just doesn't work.

URL to the xml document is right (I double checked it)

I also enabled "Show error messages." under DEBUG, but errors are shown.

The importer doesn't fire up even manually from: /import/my-importer.

How can I trace this?