I have found a way to get around this problem, by doing what the error message suggests, and using a com.google.appengine.api.datastore.Text object instead of a String. When using Text, the json should look like the following:
{
"priority": 1,
"title": "xxxx",
"date": "2013-12-17T19:30:00Z",
"duration": 120,
"tickets": 12,
"price": 1400,
"description": {
"value": "long text goes here"
},
"location": "location"
}