The only way I've managed to resolve this is to re-install and start again. Not a solution people will likely want, but it did work. This still leaves me with the company set-up settings page without permissions for normal users (only Admin can change), but this is a one off use screen, so not an issue.
Vague speculation might lead me to think that one of the Addon/App Installations only partially succeeded. But that is quite speculative.
As I've not pinpointed the actual cause, your mileage may vary...