Cookie errors

Hello,

As of the past month or so, I’ve been receiving this message:

“Error: Cookies are blocked or not supported by your browser. You must enable cookies to use WordPress.”

It’s happening on a few sites I have this plugin installed on but not on other sites that don’t have it installed.

What can be done to fix this? It’s not random, it happens after each log out and whenever someone tries to log in again.