My provider upgraded PHP to a new version, and my old Zen Cart no longer works, so I'm starting from scratch w v1.5.4

Finally got the admin directory change to work, set a new password, and now, nothing works.

I am taken to the admin page, but every action (adding manufacturers, for instance) takes me to the login page, and it saves no data.

I also get the warning that information sent is unsecured (click to continue) yet I'm on an HTTPS page. What did I miss?

TIA
Keith