I'm currently heavily involved in setting up FreeNAS and am not desperate to burn CDs but I think I can reproduce the same error of burning CDs from both FLAC & MP3 files on two separate computer configurations.
I also setup a completely separate library. As part of a test, when I get a chance, I will import my year old MP3 collection of 7K files and then try to burn from that. Unfortunately, every computer in my home has essentially identical configurations because when I format every year, I install everything at the same time on every computer so it may be some software specific to me--although my software base is not anything out of the ordinary. I use a lot of free, open-source tools.
I do have a separate computer I could set MC up on. It is an older computer with an older setup which may help me pinpoint the cause. I'll do my best to provide scenarios where the error occurs.
lol...I knew the mp3 part was important but I just didn't even remember that test I ran a while back.