I've been looking into this over the weekend, and we will see how we can investigate this further.
It appears that the domain has been delisted by Bing and subsequently is no longer returned by Duckduckgo, which uses Bing as one of their search engine sources. Such a delisting is apparently easy to cause since Bing does not validate their quality factors the way Google does, so a large number of backlinks rated poorly by Bing may have caused the domain to drop (no comment if this is "organic" or was caused with malicious intent).
Still, given the respective search volumes on Bing vs. Google and other search engines, it is an issue to be followed up on, but not of immediate concern. Google search appears to remain unaffected, and since the forum itself is an added value site and not a commercial venture in itself, it is an annoyance right now.
We'll see what we can do, but if it is indeed due to spammy backlinks on sites beyond our control, little may come from it.