Error in workflow Purchase requisition

Dear All,

I got an error in work flow PR.My  PR workflow suddenly stop. I check the history and find the error

"Stopped (error): X++ Exception: Failed to create a session; confirm that the user has the proper privileges to log on to Microsoft Dynamics.

 at "

FYI : I use AX 2012 R2.

Does any one can help me ?

Thanks in advance

Regards

Rudy.