Currently having this issue.
Was playing on a small business lot type on the Inked Inlet Lot. Opened my game today and went into build mode. I increased the size of one room, then when I tried to drag the second room over as well it wouldn't complete the action. It would let me pick up items, but not place them back down. The undo button also wasn't working.
I attempted to go to manage worlds, but my computer started sounding like a jet engine and I got the infinite loading screen. Had to close the game through task manager. This happens every time I open the game and go into build mode. Even quitting the game ends up freezing it, have to go through task manager again.
Fully disabled CC (CAS CC, MCCC, and UI Cheats are what I was using). Same thing as above.
Next tested a different lot in the same save file with CC disabled, didn't appear to have any issues, so I went to my original lot from manage worlds and entered build mode directly. This time I didn't have any issues. I'm wondering if there's an issue with entering build mode from live mode.
Tried in a fresh save as well, entering from live mode on the same lot as a small business, but didn't find any issues. It did take almost 8 hours of gameplay for the original issue to show up, though.