That's weird, 524,288 bytes is exactly 512 kb which is the size of a regular save file, but in that case it should be .sav not .dsv. Though Gen 5 save tool should be able to load it regardless of its file extension.
Let's try this, in DeSmuME go to File > Export Backup Memory and see if the resulting file can be opened. If it doesn't please upload your save file so we can check what might be wrong (I assume you already saved at least twice ingame).