Why am I getting an error when I sync my iPhone 5S?

The error message "The iPhone "iPhone name" cannot be synced. An unknown error occurred (-54)." came up when I have been syncing my new iPhone 5S.

This didn't start happening until last night, when I restored my iPhone from an old back-up I had from my iPhone 4 that I recently transisioned away from. Was this a mistake? Do I have to restore it to factory settings to get this error to go away? What do I need to do?

Also, something to note is that when I looked this error message up, I came up with some solutions for an iPod getting this same error message, but nothing with an iPhone. The solutions had something to do with a locked file blocking the process. How do I know if a file is blocked?

I have an mp3 on my iPhone that won't go away with deselecting "Sync Music," But every other music file was able to come off with clicking that button. Is that the problem? The only way I can think of getting this file off is to do a factory reset to my iPhone. I'd prefer not to do that because I lost some phone numbers in the process of restoring it this last time. Apparently my back-up was from 2013, when I could've sworn I backed it up right before I got my new iPhone.

I'd love help with this, please. Thank you so much!