Found here: https://jira.atlassian.com/browse/GHS-5425

GreenHopper was using a Date Time Picker provided by JIRA to capture input for start and end dates of a sprint. We disabled the text box because we don't want users to manually type the information (or remove it).

Since JIRA 5.1, clicking the calendar icon throws an exception, which GreenHopper displays in the big red box.

For our sake, it would be better if this was handled more gracefully. But also for other plugin developers. AFAICT, despite this bug, the control still allows users to change the date.