Password protected page opens blank .. sometimes

Several of my web site users need to access the password-protected pages and they have been reporting pages opening blank after entering the password. Closing the browser and re-opening, does open the page (without asking for the password). This has been happening for me for long time. I upgraded all my site last month with the latest WordPress. But the same old problem is coming back.
Has any body managed to resolve this. I am using sucuri firewall and caching. I turned off caching for the password protected pages. Problem seems to happen less frequently, but still happening ??