I was just trying out the new feature of creating workouts in Garmin connect.
After creating the workout, I hit the upload button, and two things happen:
1) I get a warning about not being able to send power duration and target to my device (Forerunner 610), although I hadn't set anything about power in my workout.
2) The upload fails with a cryptic error:
TypeError: 'undefined' is not a function (evaluating 'this.plugin.FinishWriteFitnessData()')
It turns out the workout name was the culprit. The name that caused the error was: 4x[800-5:21 400 jog]
By removing the brackets [ and ], I was able to get the upload to work. So it seems as if there may be some differences in what characters are valid on the device versus what can be created in Garmin Connect.
(Incidentally, if I create a workout with the name [] — just brackets — it doesn't give the error, but it doesn't upload.)