Got this issue in the morning. We have service that "aggregate" data from multiple RSS into 1 RSS. The issue appeared because at one of RSS we enabled encode option [encode UTF-8] but not [encode UTF-8 without BOM]:
encode in UTF-8
encode in UTF-8 without BOM.
our Java parse does not understand this BOM(byte order mark). So we just swtiched to one without BOM byte.
I've made tests using this online tool, it worked just as I expected XMLValidator