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This error usually means that the Guide signal had been edited (e.g. cutting it in two or more pieces) since it was last added to VocALign. Although VocALign knows that the Guide has been cut, it cannot know which bit of the cut Guide to use for alignment.  To generate  a valid aligned output, the guide and the dub need to be recaptured  (using "Drag and Drop" or the menus) before any reprocessing.