I had this problem when I installed a new version of client access on my PC.
IBM kept trying to tell me it was caused by a workstation device being
varied off that the AS/400 session that I was attempting to start was trying
to use. Now that may have been the case with somebody else but not me. I
couldn't get the problem to go away unless I put the older version of client
access on. But there was a service pack available for the new version and
when I applied it, the problem went away.