Symptoms

You are attempting to update a Sprint field using the REST API but not having any success. You are able to update all other fields just fine with the REST API. 

The syntax looks something like this:  

{"fields":{"customfield_10012":15}}

The resulting error does not provide much information:

Array{"errorMessages":["Internal server error"],"errors":{}}

Cause

 Updating a Sprint field using the REST API is not possible.  This is reflected here: 

The REST APIs implemented for JIRA Agile were not implemented with the intent of being publicly available/sanctioned

Additionally, our needs are sometimes more complex than just simple nouns and verbs. For example, updating a sprint's status to 'complete' involves a lot more business logic than just updating one field on the record representing that sprint.

Workaround