I think it's because housing got merged into the "Game Systems" category a while back.

See https://forums.elderscrollsonline.com/en/discussion/668568/game-systems-category-explainer#latest

But yeah it's sorta confusing and I'm not sure what's working as intended and what isn't... I can also still post in the ToT sub-forum, but not in housing.

I did not check if I could post on an existing thread. I just wonder why I cannot open a new one.

I posted the thread linked above and Kevin answered with this.......

"Hi all. Thank you for highlighting this. This seems like unintended behavior. The idea was to give access to still view the older housing threads, but have posting happening in the main game systems category. We got user feedback that we had too many categories, so we tried to consolidate some. But Vanilla has some odd things happening on the backend that makes interacting with the category funky.

We will look into this, but I guess the follow up question I have is, would you rather just having the housing thread back? We were trying to accommodate feedback, but if the consensus is that you would rather the housing channel back, we can do that."

So some people can make discussions and post replies but it is unintended behaviour, the way to go is to post your discussion in "Game Systems" and add the prefix [Housing] at the beginning of your thread title.

If you would like the original Housing Category back please let Kevin know, either here or in my thread!