All right, I done mended it. The problem was that 624's avatar linked to an image on doblelol.com, a site that Google has classified as being at risk of serving malware. Setting him to "no avatar" resolved the problem. 624 also has a post on the Quarantine thread, hence the issue there as well.
I disabled Fenwyr's avatar because he was also linking to an external site, although there's no suggestion that it was causing any problems. So everyone knows, when you set your avatar please DO NOT link it using a URL; do a right click on the image you want to use, save it to your hard drive, then upload it to JO using the Browse button in your profile. That way we don't have to worry about the source of the link being compromised and throwing malware warnings in this way.