I had copied some "*.ogg" files from Android to edit their properties.
However after changing the properties, the update for one file failed with some strange error message.
After that I found out that the file, even though named "*.ogg" was actually a WAV (RIFF) file. After renaming it from "*.ogg" to "*.wav" I could update the properties (metadata).
I'd wish fb2k would not falsely deduce a file type from the file extension.