After updating all the frontend files, plugins and database I get an error when I go to the Systems administration --> Notifications area (see moodle3.4 png). Most site administration navigation throws errors and I'm not able to get to the intended page. Unfortunately, I don't have enough space to build a complete replica of the production site to debug and test what is wrong with my production instance. I guess I'm hoping my problem is a known thing and/or someone could point me in the right direction.
Those settings don't exist in Moodle 3.4. So.... how did they get there? Has there been some local customisation of authentication code?
There have been no customisations other than a basic authentication plugin (auth_basic) which I was suspect of. I was not able to recreate the error on a clean installation of 3.4 when I added the plugin though. Regardless I'm not using the plugin so I've removed it from my production instance.
I have been able to fix this issue. It was indeed settings associated with the self-generated "Cadre" and "Staff" roles which were outside of the normal course and user level parameters. The definitive fix was to write new roles from the default student role, modify the settings I required for the roles, migrate the users in the old roles and then finally delete the "Cadre" and "Staff" roles which were the source of the problem. Once I did this the upgrade went without any hitches.
Hope this helps someone if they have the same or similar issue.