At least since thursday 2016-09-29, probably a bit more, but that's when I discovered it. It has been impossible to mark threads as solved. Instead you get taken to a white blank page.
Report it using the feedback tab to the right. Then you can mark this one as answered :)
Oh ok, there's a better place to report website bugs, great. :-). As you said this is also fixed now.
I just wonder now where to report feedback tab bugs... :)