Keep getting logged out on moving to new page

I have some pages on my site that should only be visible to registered users. There is a page called “members” (in the menu) and sub-pages (not in the menu to save clutter).

I tried both “Members” and “Page Restrict” plugins, but in both cases, as soon as you go from one members-only page to another, you are logged out (from the frontend and the backend) and have to log in again to see any other restricted page. Members is preferred, as I want to allow editing as well as viewing by page.

Clearing cookies doesn’t help, nor does removing “Sidebar Login”, which I thought might be interacting.

Other plugins I am using are “Link to Post” and WP Table reloaded”.

Any clues or suggestions for a WP newbie would be much appreciated.