Installer hangs at "Installing component sources"

I'm installing 5.13.1 from the offline installer, to /Developer. This is under Mac OS 10.14.6, with plenty of disk space.

It stalls at:

Installing component Sources...
Extracting "5.13.1-0qt3d-everywhere-src-5.13.1.7z"

The thing is, it prompted for permission. I'm the admin and I authenticated, so there shouldn't be a problem. It manages to install

{{Qt
5.13.1
Src
qt3d
config.tests
dist
examples
include
src
tests
tools
usecases
dist
installer-changelog}}
then nothing else.