Validating a generated XML using "xmllint", I got the following error message:
I was puzzled, because "xs:ID" can contain alphanumeric characters...mets.xml:46: element techMD: Schemas validity error : Element '{http://www.loc.gov/METS/}techMD', attribute 'ID': 'video' is not a valid value of the atomic type 'xs:ID'.
mets.xml:69: element techMD: Schemas validity error : Element '{http://www.loc.gov/METS/}techMD', attribute 'ID': 'audio' is not a valid value of the atomic type 'xs:ID'.
[SOLUTION]
The error is also thrown if the values of the IDs (in my case "audio" and "video") appear more than once.
I overlooked this at first, but due to multiple audio-video tracks being present in the source material, I actually got more than just one audio/video section.
The validator was right, but the error message is a bit confusing
So: Make sure that all values in attributes declared as type "xs:ID" are unique across your whole XML file!