Adding stairs in a certain save file "crashes" my game

For about a week and a half, I've had issues with my game whenever I try to add stairs to my builds. I can't expand them, delete them, or anything. As soon as the stairs are involved, I can't exit the game and the save screen spins endlessly. I can't use the arrows to go back before the stairs were added either. The game doesn't technically crash or freeze. I can continue to build, but since I can't save, it doesn't matter because as soon as I force the game to shut down, all the process is obviously gone.

I'm swedish and I get a small red error message that says "processgenerering misslyckad" which I can roughly translate to "process generation failed". I've searched for this error message in forums but can't find it, so I'm assuming it's translated to something else.

I do use mods and I've done everything to figure out if I'm dealing with broken mods. I've updated and done the 50/50 method, but there's nothing. I've looked at script call fails and all of that.

What's also odd is it only happens on save files from before issue started. While troubleshooting, I've created a new save file (with all the mods still there and active) and that one doesn't have this issue.

I don't know what to do or how to proceed. I want to be able to use stairs in my builds and I don't want to abandon the save files where the issue appears!