Yes this is a very common error for my client also, it seems for some reason when clients get their new Centova ,some will upload a few mp3's, then delete them all -- when they go to try and re-upload they get the disk space error. Not sure why they do this, weather they are testing it or what, but it can be a problem and needs to be corrected because new clients think there is something wrong, I even had one file a Paypal dispute
The fix is easy though, in the file manager bottom right, just click on the graph, that will sync the files up, or at least it should, I had a couple accounts where that didn't work so I just set the clients disk quota at unlimited