Presently in the menu, Install Release (oh well I have 'Installer RELEASE' in french) have: sh -c 'ubiquity gtk_ui'
I have done today and surely got the ubiquity bug near end installation again (same lines and messages at end of journalctl, but the report was said to be corrupted.
I suggest it become 'gksudo ubiquity gtk_ui' as a workaround the bug I described in an other post near end installation when installing in a non english language (well, did not really try english).