To the clan thing, I wasn’t in a clan when I started building my base (since its just the two of us on the server) but I made myself a clan when I started encountering this problem to see if it would fix it. So being part of a clan isn’t the issue, and as it impacts such a large area with my base only being near the bottom of the image I posted, it shouldn’t be that, since it doesn’t say ‘Land is claimed’ either while it does when I try to build near my friend’s base.
We didn’t adjust LandClaimRadiusMultiplier whatsoever and I don’t think it could be that since I can build fine INSIDE of my base at the southern end, but there’s this horizontal border that I can’t build north of. Here’s the line. The problem started when I just randomly couldn’t build anymore while building that house to the right. https://i.imgur.com/TzyW1nq.png
My friend has the same issue as me as I asked him to try and build something north of my base where land wouldn’t be claimed, and he said he couldn’t build anything there either. Luckily for him, his base is south of mine.
EDIT: For the record, the LandClaimRadiusMultiplier on the server is set at 0.5