Workflow engine access denied

Hello,
since few days I get a this strange error when administrator's group members make changes and wants to publish them with workflow manager:

Error :

Error during handle of operation, Error during handle of operation, root cause:Workflow engine access denied, User 'guest' has no permission to acccess this engine


Exception StackTrace: Cause level : 0 (level 0 is the most precise exception)
org.jahia.exceptions.JahiaException: Workflow engine access denied, User 'guest' has no permission to acccess this engine
	at org.jahia.engines.workflow.WorkflowEngine.handleActions(WorkflowEngine.java:204)
	at org.jahia.operations.OperationManager.handleOperations(OperationManager.java:302)
	at org.jahia.bin.JahiaAction.execute(JahiaAction.java:50)
	at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:421)
	at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:226)
	at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1164)
	at org.jahia.bin.Jahia.process(Jahia.java:1547)
	at org.jahia.bin.Jahia.service(Jahia.java:862)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:809)

I'm using Jahia 4.1 build 6050

Thank you for help and support.
Best Regards

-- 
Lorenzo Patocchi
Dipl. Ing. STS/ETS/HTL
 www.cryms.com
Tel: +41 91 60 55266
Cel: +41 79 375 0891
Fax: +41 91 223 9797

Cryms sagl
Via Cantonale 34a
6928 Manno - Lugano
Switzerland