Hi lmoran,
there were two errors in the service, an old one (the error you posted) and a variable I forgot to rename when I've updated the service for goText++ Desk! 0.9.0.
I've updated the service, please do right click => update the service, or run the massive update from Help => Check for updates, or download the updated xml if you added the service from file.
Let us know if it is working fine now.
PS: I also see a mistake I did in goText++ Desk! code, there should be a step name instead of the %2 .. I'll fix this minor problem for the next release.