AX Workflow freezes

Hi, we are having problemas with our workflow on dynamics AX. It freezes at some point. Event Viewer reports the following:

Dynamics Adapter CallStaticClassMethod failed.

SysWorkflowWorkItemService-createWorkItems        SysWorkflowEventDispatcher-onWorkItemCreate    Error en la solicitud de permiso de tipo 'ExecutePermission'.
(S)\Classes\ExecutePermission\demand
(S)\Classes\DictTable\callObject
(S)\Classes\EventNotificationWorkflow\addCustomMergeValues - line 13
(S)\Classes\EventNotificationWorkflow\sendMail - line 28
(S)\Classes\EventNotificationWorkflow\create - line 24
(S)\Classes\SysWorkflowEventDispatcher\onWorkItemCreate - line 323
(S)\Classes\SysWorkflowWorkItemService\createWorkItems - line 162


Microsoft.Dynamics.BusinessConnectorNet.BusinessConnectorException
   at Microsoft.Dynamics.BusinessConnectorNet.Axapta.CallStaticClassMethod(String className, String methodName, Object[] paramList)
   at Microsoft.Dynamics.BusinessConnectorNet.Axapta.CallStaticClassMethod(String className, String methodName, Object param1, Object param2)
   at Microsoft.Dynamics.Framework.BusinessConnector.Session.DynamicsAdapter.CallStaticClassMethod(String className, String methodName, Object param1, Object param2)

Any hint about this? Best regards,