Just from looking at the code that message seems to come from the DS webservice. If you can download and run the latest version of fiddler (v4) on the workflow server it should catch web service calls as well. If we can see that its a problem with the API then the DS folks should be able to help.
Another option is to try and find the IDs in the DS database and not use the pickers. maybe that will get you around the issue.
Fiddler: http://www.fiddler2.com/fiddler2/version.asp