Winamp - Cannot save metadata: Unspecified error

When trying to save ID3-data mp3 file in Winamp you may encounter the following error:

"Cannot save metadata: unspecified error"

Then when looking at the ID3 data you may find that the ID3v1-data was 
written, but not the ID3v2-data.

The solution for me was to ensure that the directory(!) that the file was in 
had writing permissions. For some reason you can write ID3v1-data but not 
ID3v2-data when the directory is locked but the file is not.