After checking 1.0.1 of my MOD I get an

Validation results: Validating zip: ./phpBB3/store/b0300a1d73a4fc783ecfeafb6265785e.zip
[ FAIL ] install_custom_header_logo_1.0.1.xml: Invalid XML/MODX

Element '

Unknown macro: {http}

mod': No matching global declaration available for the validation root.

I included the new MODX 1.2.1 and referenced the xsd shown in the updated MODX sample file

<mod xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.phpbb.com/mods/xml/modx-1.2.1.xsd">

Here's the uploaded MOD:
http://phpbb.krizsan.de/custom_header_logo_1.0.1.zip