Hello Jonathan,
Are you trying to edit a Custom Organ Design Module organ definition (CODM ODF), or a 'full-format' ODF? Whilst we don't officially provide support for editing the latter (except to major sample set producers), if you want to paste an excerpt into your reply showing the relevant line number and a few lines either side, along with the specific error text (which you could copy/paste from Hauptwerk's log -- "View | Activity log") maybe we can spot the mistake.