My friend, it is somehow a server issue meaning it is now limitation on disk space but limitation of the files that can be included in a folder, such as the cache one in media/k2/items...
That can be resolved by "telling" k2 to store images in separate folders for each size or in separate folders every month, eg 082013, 092013 and so on....
But I cannot find how to do it in the code... so I urge for the help of anyone that can assist me on that.