No, it was definitely the song file with an SGU expension. It shows with ".SGU" when Windows extensions are enabled or disabled. ".SGU" extension is not recognised in my Windows as being related to any particular application and so it does not come under the rules applying to enabled/disabled extensions.
This is a problem that PGMUSIC should have dealt with years ago, but hasn't. It is paricurly nasty because of the unpredictable behavious that results.
In RealBand this is quite bad. In my workflow I had decided to arranged my songs in numerical order such as 1.Song1, 2.Song2 etc. When I saved the song as an SEQ I got terriible results when retrieving a song, because RealBand thought that anything following the first period was the extension. I informed PGMUSIC at the time and decided not to use periods in my file names. Now, in BiaB it has raised its ugly head again due to the extensions being hidden.