Move your Mods folder to the Desktop and delete localthumbcache.package. Then test the game in a new save.
If it works now check for updates to your Mods and CC. Add it back a little at a time while testing until you have found all that are broken.
I have no mods and I'm having this same issue, only with 2 apartments so far. I can't get in to perform maintenance.
I am also having this issue. I have several lots, with units ranging from 1 to 4 per property. I have 1 lot where I am unable to access only 1 unit. No matter how I travel to the property and can never access that unit. Clicking the door (if it shows up, there is a glitch that causes doors and windows to appear as blank spaces about 80% of the time) only yields the option to change control to that household. I am unable to do normal maintenance or any event on that unit only. I have no mods and I am on PC.
