There is no way how you could ever convert *.vbox files to *.vdi files, regardless if they both come from VirtualBox. A *.vbox file is just some kind of settings for the virtual machine, so just a one piece of a whole and it needs to be converted as a whole, typically for other virtualization platforms.
This conversion entry was verified by us and conversion was flagged as wrong or not possible.
Usually this happens when the conversion would try to convert different types of data between each other, such as audio to image, or it is not possible because incompatibility between programs, caused by closed file formats, very limited support etc.
If you find entry that you feel is flagged as bad wrongly, please tell us in the discussion below with as much as information as possible so we can fix it.