OData issue in Project Server 2013

Hi everyone, I have a problem, I create a timer job that
execute a C# code get the tasks user information to modify some data, but when
I trying to save the modifications it shows me this error

<?xml version="1.0" encoding="utf-8"?><m:error xmlns:m="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata"><m:code>-2130575251, Microsoft.SharePoint.SPException</m:code><m:message xml:lang="en-US">The security validation for this page is invalid and might be corrupted. Please use your web browser's Back button to try your operation again.</m:message></m:error>

The way that I trying to save the modification is this

context.BeginSaveChanges(new AsyncCallback(r =>
            {
                try
                {
                    var dsr = context.EndSaveChanges(r);
                    if (dsr.Any(op => op.Error != null))
                    {
                        //Show error message
                    }
                    else
                    {
                        //Succeeded
                    }
                    _finish = true;
                }
                catch (Exception ex)
                {
                    _finish = true;
                    Console.WriteLine("Error Result: " + ex.InnerException.ToString());
                }
            }), null);

            do
            {
                System.Threading.Thread.Sleep(1000);
            } while (!_finish);

The error shows in this part:

var dsr = context.EndSaveChanges(r);

I hope you can help me, I can't find a good source to trying to modify the information.

Regards.

.Net Developer and SharePoint programmer