I attempt to use the SOAP updateIssue method to update a custom field. This fails with this exception:

<Fault soapenv:Server.userException:
  com.atlassian.jira.rpc.exception.RemoteValidationException:
   Fields not valid for issue: 
   Errors: {assignee=User 'XXXXX.linden' cannot be assigned issues.}

Indeed, XXXXX.linden is a disabled account - but I'm not assigning the issue, the issue is already assigned. Effectively, your validator seems to first merge my updates with the current state, then run a validation, and then notice that it's assigned to a disabled user and fail.

Re-assigning the issues is not really an option, as we would like to see who was the last assigned person on closed issues.

Here's the actual code, using python soappy:

            try:
                soap.updateIssue(auth, id_string,
                    [ { "id": "customfield_10310", "values": values },
                      { "id": "description", "values": description },
                      { "id": "environment", "values": environment } ] )

So nowhere do I actually attempt to reassign.