by Emma Richardson.
It appears to me that the problem occurred in 4.1 but you did not deal with it there and continued on upgrading a site with problems to 4.3. I would suggest returning to 3.11 (hopefully you have backups) and then retrying the 4.1 upgrade. Before upgrading, I would check all plugin compatibility and remove any additional themes and switch your site to boost. 4.x brought in a total ux overhaul and I would guess that maybe you have a theme or plugin that is not compatible with newer versions.