Two issues with multisite setup, first, when a user tries

Two issues with multisite setup,

first, when a user tries to sign up for a site they get a message saying "wordpress is already installed, would you like to reinstall", but the new site is generated in the back end??

Second, in subsites, when trying to enable a different theme, everything goes blank. Front end as well as backend. (on subsite level) It is not possible to go back. Have tried to reset permissions, but that does not work.

What causes this?