Hi, i just find and error on CRM online 2013.
The steps to reproduce are:
1) create new custom entity
2) create new sync workflow in before delete and add a step in the workflow. leave the "Execute as" as the default value ("The user who made changes to the record")
3) publish all and activate the workflow
4) create and try to delete a new custom record
5) throws the error (see bellow)
6) if i change "Execute as:" field in the workflow to "The owner of the workflow" it works fine.
7) if i change again to "The user who made changes to the record" it works now.

The full error description is:
Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=6.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: System.Web.HttpUnhandledException: Microsoft Dynamics CRM has experienced an error. Reference number for administrators or support: #9BCAE0E7Detail:

-2147220970

System.Web.HttpUnhandledException: Microsoft Dynamics CRM has experienced an error. Reference number for administrators or support: #9BCAE0E7
2014-02-05T20:30:29.5087838Z

-2147220891

Value cannot be null.
Parameter name: culture
2014-02-05T20:30:29.5087838Z



[Microsoft.Crm.ObjectModel: Microsoft.Crm.ObjectModel.SyncWorkflowExecutionPlugin]
[4b68b225-a48e-e311-b0ae-6c3be5be5e68: ]
Starting sync workflow 'test', Id: 4568b225-a48e-e311-b0ae-6c3be5be5e68
Entering ConditionStep1_step:
Sync workflow 'test' terminated with error '