I finally got around to working on the issue - resolved. Comparing the Moodle 3.4.3 files on our server against a fresh install... The issue with the Boost theme was related to:
- boost\templates\core\single_select.mustache
- boost\templates\core\url_select.mustache
In addition these files should not exist:
- boost\templates\core\logn.mustache
- boost\templates\core\pix_icon.mustache
- boost/pix/fp/*.png
All these files are from 9 July 2017 so Moodle 3.3.x branch. We did the Installatron upgrade.