Hi,
The first 2 lines are part of the server code; they do not use or depend on AD plug-in. By 'does not work anymore' do you mean that the code throws some exception, or that is always returns null/undefined?
For the second part - the error you are getting means the code is trying to convert object of type byte array [B to type String. Do you get this error for the attribute userPrincipalName or for the attribute that is enumerated after userPrincipalName?
Could you show your scripting code? Also, could you check in your AD what is the type of the attribute that trows the error? (In my sample AD, it is of type String and Burke's sample code works just fine for it.)