Cannot change theme from default_simple or view pages

[Patr100]

Hi
I just installed GS - health check etc OK
but when I choose and activate the alternate theme (ie Cardinale )
I see the preview image etc and press "activate" it says
"changed successfually3 but it hasn't
- I am stuck with default_simple and cannot change to Cardinale

also if I select to view any created pages wiith the present theme I get
"Not Found
The requested URL /index.php was not found on this server."