I've experienced this "malvertising" twice while the forum was open (using Windows 10, Chrome with ABP). I don't recall the details, unfortunately. Too concerned with shutting the browser down without accidentally clicking something on the ad and triggering some software install, etc. I also had other sites open, so there's no way to be certain of the source.
I would agree that IF it is coming through this forum, it would most likely be through the advertising. Google search for advertising malware news and you'll find several instances of major ad networks being compromised over the last year or two. It would fit the intermittent nature of the reports, the lack of evidence of any malware in the forum itself, and the inability to reliably replicate the issue. Pretty tough to fight this type of problem from the site side I imagine. The ads rotate and the malicious code may be capable of detecting certain system settings or hypothetically even identifying individual systems to avoid repeat occurrences. Oh for the internet of my youth when the malware was written by trolls instead of organized criminals!
Edit: to the best of my recollection, this was before I signed up for the site, so maybe related to the increased ads shown to guests.
Edit2: Ran SpyBotSD which found nothing of consequence, and MalwarebytesAM which found one likely threat. If the date on the folders where it resided is accurate, it was installed before I bought the computer (used). This "softup.exe" appears to be an adware trojan which is scheduled to run periodically to serve up ads. No way to know if that's what was at fault, but certainly a possibility, and I'm glad to have eliminated it from my system.
