Additional info: I partially solved the problem. While the path and the file names was quite short, the mp3 had lot of long tags inside. I removed the tags with MP3Tag software, and after then I did the File > Scan Folders for New Files operation.
This time, the operation went good, so I think the long tags were the problem. However, I'd still love to find that "disappeared" Log menu. Maybe in that log I could had find the solution with no wasted time.
Thanks again for the attention.