zase soubor *.zip nelze otevřít jako archív.

Začínám si myslet, že ty zip archívy vytvořené cobianem jsou nějak nakoplé.
Studuju jejich tutorial tady http://www.cobiansoft.net/?cat=3 a píše se tam, že

Compression is normally discouraged (at least when you are NOT compressing individually). Compression results often in very fragile backups. A single error in one byte of the archive could result in a whole backup going bad. And a lot of things can go wrong with compression, believe me.