OK --- At the beginning of your template you probably have something that looks like this:
$iso = split( '=', _ISO );
echo '<?xml version="1.0" encoding="'. $iso[1] .'"?' .'>';
The change that I would suggest as a first attempt to repair this is to replace that code with the following:
echo '<?xml version="1.0" encoding="ISO-8859-1"?' .'>';
Hopefully, that takes care of the issue