Place Bid button leads to Oops! That page can’t be found.

Half way into creating auctions for a Lego sale I’m having, several auctions suddenly started failing, where when you try to add a bid it simply takes you to a page that says:

“Oops! That page can’t be found.”

I’ve tried resetting my permalinks, but the fact that it happens at the Place Bid stage is weird – the permalink has already been called (the Post that houses the WP Auction), and especially weird because other auctions are behaving fine.

I’ve tried comparing a successful auction with one that fails, and I can’t see any differences in the Auction, the content, the Post or anything.

Help!

The page I need help with: [log in to see the link]