When executing fc-blanks, it tries to download a list of blanks from the UnicodeSet Utility , but the service is currently down, only returning a message saying "This page has been disabled, pending resolution of unusual load problems.".
The page is downloaded, but, as the content is different from expected, the build fails in line 37.
Workaround: turn off network connectivity, so the download will throw a HTTPError exception and the build will use the static data on list-unicodeset.html.
Hmm, we could have a workaround but that is what they have to fix... surprised they didn't return any HTTP error code on it.
Fixed in git