I have had the same problem.
File and folder names may be OK, but check them anyway (characters, length).
Then check for corrupted audio files,
either with Foobar component 
foobar2000: Components Repository - File Integrity Verifier
(“verify integrity”)
or any software able to do that.
…like this one “audiotester” in vuplayer.com
Before discarding these files, keep the list somewhere…to re-download (or rip again) them …if you really need them 
Be patient !