it sounds like this is an issue with character encoding. Sometimes this has to do with older php version or some other setting, but I cannot be sure as I don't have access to that information on your server.
One thing you can try, is editing the plugin file aws_signed_request.php on about line 40, to parse the XML in the same encoding you have for your site (UTF-8).
So it currently is:
$parser = xml_parser_create ('ISO-8859-1');
Change it to:
$parser = xml_parser_create ('UTF-8');
And see if that helps.
We have been testing different options for the next update, and it might be that we change this to automatically be the same encoding specified in the
bloginfo( 'charset' ) call - but not everyone uses this to actually output the encoding type to the browser or they have the DB_CHARSET defined differently in the wp-config file and use a conflicting type in the meta charset in the browser head (because they hard-coded it).
Hopefully that will work for you.