Login only functions when using Private Browser

Hi,
Bit of a strange one, my login function only seems to work when using Private Browser mode. I’ve tested the site on 2x mobiles and 3 desktop devices all the same issues. Firefox, IE, Chrome whatever browser unless on private mode it won’t login.

Also, presuming there’s a link between the two things, when creating a new Administrator account in the Users section, and then trying to login with that user, it generates the “You do not have sufficient permissions to access this page” error rather than showing the Admin Dashboard.

Thanks,