My problem is described here: http://qt-project.org/forums/viewthread/46181/
Short: The binarycreator.exe does not remove the read-only flag from files it copies to (or creates in?) the temporary directory and then fails building the installer exe. This is due to the files/folders not being writable and deletable.
If it does not fail on the first build, it does on the second one, because the files are already there, but again can not be written...
Same behaviour with the official 1.5.0 build and this one http://download.qt-project.org/snapshots/ifw/1.6/2014-06-19_66/installer-framework-build-win-x86.7z