I suspect that the data context does not like being accessed from multiple threads.
I recommend that you first remove all processing from your extension method:
public static Task<IEnumerable<TResult>> ExecuteAsync<TResult>(this DataServiceQuery<TResult> query)
{
return Task.Factory.FromAsync(query.BeginExecute, query.EndExecute, null);
}
Which you should use without jumping onto a background thread:
public async Task LoadDataAsync()
{
_ctx = new TheContext(the URI);
_dataSource = new DataServiceCollection<TheEntity>(_ctx);
var query = _ctx.TheEntity;
var data = await query.ExecuteAsync();
_dataSource.Clear(true);
_dataSource.Load(data);
}