Forums won't stay logged in anymore

For some reason the forums won't keep me logged in anymore after closing the browser. I don't have it set to delete cookies, and these forums are the only site its happening on. I make sure to click the remember ID box every time, but it still won't work. I've already tried deleting the cookies and such and logging back in, as well as a different browser, and both of them have the same behavior.

System Details:
OS: Windows 10 Home
Browsers: Firefox 64.0, Chrome 71.0.3578.98
Site: These forums
Date/Time: Ongoing since about the beginning of December 2018