Workflow Activity "Lookup Value" returns An error occured while enumerating the filter using [//WorkflowData/customvalue]

I want to generate an Accountname using EmployeeID, FirstName and LastName via a workflow.

I'm using the Granfeldt Workflow Activity Library (https://fimactivitylibrary.codeplex.com/)
I'm using the FIM Powershell Workflow Activity (https://fimpowershellwf.codeplex.com/)

Steps:

Passing the EmployeeID, FirstName and LastName to the powershell Activity, generating a logonid based on logic.

Add-PSSnapin FIMAutomation
$EmployeeID = $fimwf.WorkflowDictionary.EmployeeID
$Forename = $fimwf.WorkflowDictionary.Firstname
$Lastname = $fimwf.WorkflowDictionary.Lastname

'logic creating a custom logonid here

   ==> This works

Returning data back to the workflow via that powershell script:

$fimwf.WorkflowDictionary.Add('NewAccountName',$newlogonid)

   ==> This works

 Using Lookup Value Activity to read the Workflow data and update the [//Target/AccountName] fails.

This gives an error:

       An error occurred while enumerating the filter 'string' .

(where string is the actual generated userid that I've got back from the powershell script. Example dab2563)

I tried with only [//WorkflowData], then this gives the error:

      Index was outside the bounds of the array.

Any hints to solve this?    

Kind regards,
David