Add a new header to the response:
Content-Type: application/json; charset=utf-8
But SoapUI uses the encoding of the OS. So save your response with an Editor in UTF8.
Then open the response with an Editor by using your Codepage (cp1252 or iso-8859-1, i used VisualStudio [open with encoding]) and copy paste it to SoapUI.
set the header to the Codepage used by SaopUI
Content-Type: application/json; charset=iso-8859-1
This helped me with umlauts. Without setting the Content-Type i get the error: Invalid character