No ID for acoustic guitar in Instruments.xml
Starting on line 9944? of instruments.xml here is the definition for the Acoustic Guitar
<Instrument id= > <longName>Acoustic Guitar</longName> <shortName>Guit.</shortName> <description>Acoustic Steel-strung Guitar</description> <musicXMLid>pluck.guitar.acoustic</musicXMLid>
Notice there is no ID assigned to it. I would suggest that "Acoustic Guitar" be assigned. I may have a slightly old version of Instruments.xml loaded so the line number might be different but the ID is missing. The problem with this is that it makes the instrument unavailable to plugins.