Network installation giving errors when viewing or adding a user

I have just installed a fresh version of WP which I have turned in to a network install (using the instructions found here).

All seemed to be going well, and I was able to add a new site successfully, however I am experiencing issues with listing/adding users.

When I click on the Users menu item the list table gives the following warning -

Warning: array_keys() expects parameter 1 to be array, boolean given in {my-dir}\wp-admin\includes\class-wp-users-list-table.php on line 307

This line is - $editable_roles = array_keys( get_editable_roles() ); - the issue is that get_editable_roles() is ruturning false.

The same problem also occurs when I try to add a user, although the warning is at a different location -

Warning: array_reverse() expects parameter 1 to be array, boolean given in {my-dir}\wp-admin\includes\template.php on line 843

I've tried the obvious, deactivating all plugins (I'd added one) and switching to the default theme, but the warnings still remain.

Can anybody please suggest how I may fix this problem?