I'm not sure that I'm getting exactly the same problem as
IMH, but it's similar.
If I go directly to
https://www.flyertalk.com/forum/usercp.php the white box appears with buttons (including the Save and Exit, Reject All and Accept All buttons at the bottom of the page), but none of them are clickable - every button is dead. The Manage Settings link is also dead.
However, if I go to
https://www.flyertalk.com/ then all of those buttons work. This means that I must go to the home page first, do the cookies stuff and only then can I go to
https://www.flyertalk.com/forum/usercp.php.
I am usually using Chrome on Windows 10 Pro, but I get the same behaviour with either IE or Edge. However, when I use Firefox and go directly to
https://www.flyertalk.com/forum/usercp.php, the white box appears in full and the buttons work.
I have also encountered a similar problem if I'm trying to read a longer thread when not logged in. The white box floats somewhere above the top of the page that's being displayed. However, if I scroll up towards the white box, the previous part of the thread loads and the white box floats up and out of sight again. It makes me feel like I'm suffering the punishment meted out to Sisyphus.