@crinrictToo late. Already uploaded to gallery. And I just checked and The Summer Home already in my gallery also has this problem in the upstairs hall.
Also, I've been experimenting on my poor Sim, and the issue seems to be fences. Walls are fine, but as soon as I replace them with fences, he can't get there from here.
I'm not a builder, so please disregard if these are unhelpful.
I don't build, but I do download lots of beautiful builds from the Gallery. Occasionally I find that my Sims can't traverse certain areas.
The most common problem is objects placed with "moveobjects on." For example a large decorative plant placed next to a bedroom wall makes it impossible to access the bathroom.
Other common obstacles are lights. Especially ceiling lights. A large ceiling light in a hallway may prevent my Sim from traversing the hallway because they can't walk under it.
The weirdest one is doors just don't work. Sometimes deleting the door and replacing it works. Sometimes it needs replacing it with a different door. Sometimes I have to move the door to a different wall.
I once downloaded a really lovely house that my Sims simply could not navigate and I couldn't figure out why. As a matter of curiosity I saved it room by room and reassembled it on a different lot. It worked fine. The only thing that I did not copy was the pool. 🤔
