There's a new thread in the steparenting topic that has a glitch in its url.
The thread called "who pays for contact?" doesn't open - and the embedded url for that thread title just opens up the step-parenting topic index. I did a "copy link url" and the text of the thread title has www.mumsnet.com/Talk/stepparenting/$thread.ThreadUrl as its url and I think the $thread.ThreadUrl bit is supposed to be substituted but some bit of code somewhere is broken.
I got there from the "unanswered threads" link because the "trending" and "new" options on the active threads page seem to have disappeared, but the same issue is there if accessing from the topic index instead.