iOS SDK: findObjectsInBackground query returns strange JSON error

In Swift, I am creating a query based on a registered PFObject subclass. The query is pretty basic:

let query = Game.query()
query.whereKey("playerID", contains: facebookID)

Then I call for objects to be found in the background:

query.findObjectsInBackground { (objects:[PFObject]?, error:Error?) -> Void in
    if error == nil {
        print("Received objects")
    } else {
        print("Error: \(error.localizedDescription)")

And I receive the error message:
JSON text did not start with array or object and option to allow fragments not set.

I can see my data in the server and the query should return data.

I can’t work out why I would be receiving that error message. Any ideas?

I am using Parse-SDK-iOS-OSX version 1.19.1.
Additional info: I have attempted to install Parse dashboard on my Parse server to try to see logs, but it doesn’t work for me.