The "(ERR)" means that Rockbox couldn't play the file for some reason. Either the file couldn't be opened, or perhaps more likely, there's something wrong with it, e.g., Rockbox not being able to read the necessary metadata.
Copy the file(s) to a computer and check it there (e.g., trying to play it in some program). If it works there, one of the Rockbox developers need to take a look at it to determine what the problem is.