I am having the same problem when any 'latin' characeters are sent (x80 - xFF). It appears the message sent from Paypal is encoded using windows-1252 rather than UTF-8. When the receiving website attempts to translate the string into JSON it causes an exception since it is expecting a UTF-8 encoded string. From the Profile> Selling Tools page I have selected the 'paypal button language encoding' link and then pressing the 'more options' button updated the encoding option to UTF-8. Unfortunately the meesage sent still appears to be encoded using windows-1252.
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