Why can't I login?

One of Gwulo's long-term contributors recently got in touch to say they couldn't log in to the Gwulo website. They'd enter their usual username and password, and they seemed to log in without any errors, but the "What's new" page didn't show anything as new, and if they tried to add anything to the site (eg add a comment, upload a photo, etc.) the website asked them to login again.

A screenshot showed that at the bottom of the Gwulo web page, there was a blue bar showing "We use cookies on this site to enhance your user experience". I asked the contributor to click on "Ok, I agree" on the right of the blue bar, then after that they were able to login and use the site as normal.

(The Gwulo website uses a cookie to record that you are logged in, so until you allow the site to use cookies it can't remember who you are or that you've logged in, and so asks you to login again and again.)