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This may be because it's trying to return a newer representation of the compressed geometry than the your route task knows how to deal with. The server can return the older format if the output routes don't include Zs or Ms. Does your network have Zs. If not, is it trying to return Ms? You can try explicitly setting the routeTask.outputLines = TRUE_SHAPE so it doesn't return measures. Is it possible for you to run fiddler or something to intercept what the route service is returning?