"Unicode character 0xfeff is a BOM (Byte Order Mark), and not part of any valid Unicode character page. Some systems may emit this value at the beginning of a file, but Apex doesn't recognize those bytes, causing the problem you see here. As a quick fix, remove those values from the beginning, if present:
Note: if you're starting with a Blob already, don't use Blob.valueOf. Alternatively, fix the source to exclude these two bytes, if possible (some text editors include the ability to emit or omit the BOM, depending on a setting or flag)."
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Recommend reviewing solution listed here (https://salesforce.stackexchange.com/questions/315800/system-jsonexception-unexpected-character-%C3%94%C2%AA%C3%B8-code-65279-0xfeff):
"Unicode character 0xfeff is a BOM (Byte Order Mark), and not part of any valid Unicode character page. Some systems may emit this value at the beginning of a file, but Apex doesn't recognize those bytes, causing the problem you see here. As a quick fix, remove those values from the beginning, if present:
String fixedString = EncodingUtil.convertFromHex( EncodingUtil.convertToHex(Blob.valueOf(sourceString)) .removeStart('feff').removeStart('fffe') ).toString();
Note: if you're starting with a Blob already, don't use Blob.valueOf.
Alternatively, fix the source to exclude these two bytes, if possible (some text editors include the ability to emit or omit the BOM, depending on a setting or flag)."
If this information helps, please mark the answer as best. Thank you