Since the FAQ portion of the is presently only being actively logged in to by one person (Steve), I've just set the Wiki software to default to traditional HTTP for now.
The previous certificate was supposed to be good for 30 days and renewable by me twice more while other options were explored. Paying for an SSL certificate for just the largely read-only FAQ doesn't really make much sense - but long term, I'd like to get the entire site switched over to HTTPS so it's something that'll have to be addressed.
But in the meantime, the error should be nominally gone (exception: any bookmarked links that current point to the HTTPS version of the FAQ will still generate the error)