If it helps: I narrowed it down. I only get those malware adds on a very few threads - others are fine. One I could clearly identify (it happens every time when opening it) is this one:

China 24, 72, and 144 hour Transit Without Visa ("TWOV") rules master thread

(I click on "newest unread post", it shows the thread, then after 2-3 seconds the redirect happened. Happened yesterday and today to me, obviously with a different "unread post" to jump to)