It works during a regular mvn install, but it fails during release:prepare. The bug is somewhere in the maven-html-to-pdf plugin. I hope a mock release using mvn -X will reveal the problem, but would take a frustrating amount of time. For now the available info is the output:
The error points to a DOM related issue, but since the downloaded manual gets overwritten with the dummy file, I have no idea if the download got interrupted, or in what way the source gets corrupted.