My chairs are not working

I love playing in the Udon flight sim worlds and messing around with avatars with chairs, but not too long ago, they just disappeared and are no longer working. I can’t sit in them anymore. It’s like they were just removed from the worlds and what not, but I know they aren’t. Anybody have an idea what I can do to fix this?

Edit: Portals are also non-functional

Try validating game files and disabling mods if you have them.