Good attention grabber with the topic title.

That shouldn't be happening at all. Occasionally, on boards that display ads, there'll be a bum ad that will have issues (popups or malware). But this board is ad-free.

Does this happen every time you log in, or did it just happen once?

You probably have something lurking on your computer that needs to be removed. While both Brewerfan and LambeauLeap are ad-free, Yuku does load a bunch of user tracking stuff. One of those trackers might have a "hook" which ties into malware already present on your computer. Of course, even if you do have malware, Yuku shouldn't be tying into it and triggering popups.

What browser are you using? Do you know how to take a screen capture? If you can grab a picture and the URL of this rogue popup, it might be useful to Yuku. The URL isn't likely to be displayed in the popup window itself, but it should be retrievable through your browser history.

I'd zero in on the anti-virus thinger. If you don't have it, I'd start by downloading and running Malwarebytes' free version. The free version doesn't offer the ability to run automatically, which means that it won't fight with any other anti-malware software you might have installed.

If you find any information that might be of interest to Yuku, consider that "extra credit." If your problem goes away, that's the main thing. However, if things persist, you may benefit from working directly with Yuku staff on the issue.

Also, if you need additional help attacking you malware situation, feel free to start a thread in Off Topic over at Brewerfan. You'll have a wider audience over there.