One of my oldest couples in Sunset Valley (one of those saves from 2016 to which I returned a couple of months ago) are suddenly acting strangely. They are settled, stable, affluent, both have achieved their life wishes, one now has a wish to be a millionaire ...which will probably be fulfilled soon. Nothing to explain this sudden rash of weirdness.

First, the sim who does most of the gardening now wants to take a circuitous route to the garden, gets stuck (apparently wants to go through a bush rather than walk around it) and stands the waving her arms and screaming about no route. Normally they just go out the back door (kitchen) and they're in the garden; now they go from the kitchen through the dining room, through the hot tub room, through the "lounge" (bar and restroom adjacent to the pool), exit via the side door, turn left and complain of the bush there instead of just walking around it. So I click on the ground near the pool "go here", click a couple of feet to their left "go here", then it's a straight line to the garden (perhaps eight tiles from where they started). There's nothing wrong with the routing in the kitchen, neither sim has any issue when cleaning the house (the trash can is just outside the back door).

Second, the other sim wants to read something, the icon of a particular bookcase appears in the upper left, they go to it, stand there a minute and that icon is replaced by one for a different bookcase, and they go there; repeat for every bookcase in the house til they're back where they started. Continue the entire afternoon, doing laps around the house, bookcase to bookcase. My first thought was they had already read everything in the house, so I replaced about half the bookcases with different ones (I have the "no reread" mod), but that didn't help. So at these times I send that sim to the library; don't have to accompany them, the minute they walk through the front door they want to "read something", and will happily spend the rest of the evening there. (Though they rarely get to finish a book, every sim in town has to come strike up a conversation with them.)

Third, can sims normally climb through windows? Lately when my sims go to the sauna they no longer take the easy way - go through the dining room and you're there. They go out the front door, walk around the front of the house to where the cars are parked (facing the wall outside the sauna), go up to the wall, lean through the window and turn on the faucets, change clothes and step through the window into the tub; when done, they step back through the window into the parking area, change clothes, walk back around to the front door, and come back inside. I put different windows in that room but that had no effect so returned to the originals.

Now, the only thing different ...the only thing new (sorry, ancient song reference)... is that I completed the fence around the lot and put in two gates, locked to all but household. This leaves the front door area and parking area as the only sections of the lot open to the street (it works, recently I heard yelling outside the house, there was some unknown sim waiving his arms and screaming about no route, then ran off down the street).

Old saves got somewhat corrupted over the time and they needs a fresh start.
You can save it as with other name to create entirely new save files with all your proggress over the years, cleaned from cache that has been accumulated. In doubt you can use save cleaning tool to clear out the "garbage" files stored in the old save (remember to do a backup folder).

About the bookshelf you got to replace it with new one, delete the old one and clear the stored books from the family inventory. When selling items from family inventory you gotta sell them all to keep it clean, because when it's full it can make the game to lag more.

Could you provide some screenshots of the fenced area around the house, the gate and the floor plan of the house's ground level?

The routing issues could be because of stuck jigs perhaps? If you use Nraas mods you could try resetting the lot, if that doesn't help then flush any stuck jigs.
https://www.nraas.net/community/post100274#p100274

Be sure to backup your save first (save as a different name) so that you can go back if anything breaks.

Thanks. I've been playing those old (2016) saves for a couple of months now with no major issues (each threw the "some items could not be loaded and were replaced" on first load, but whatever it was must be inconsequential since I noticed nothing changed; in fact the only thing noted was the aquarium was missing from all homes, the mod was inexplicably missing from the packages folder, and searching online did not disclose it, so I used one of the standard aquariums instead). The anomalies noted above just appeared a couple of days ago.

Each active save gets treated with Regul Save Cleaner about once a week.

Will try to get screenshots (if the game cooperates, it has a mind of its own regarding view angles and zoom).

"If you use Nraas mods you could try resetting the lot, if that doesn't help then flush any stuck jigs."

Interesting. Does this affect the game in any way or just clean up things in the background?

The "some items could not be loaded and were replaced" message appears when you have uninstalled or corrupted CC, store, or EP-related items that were removed from the game before opening the save. This happens if they were placed inside the current lot, somewhere around the town, or were sim related CC. That's why the game replaces them with base game items or eliminates them completely.

Edit: For this reason I rarely use custom content in my builds (no matter how many items I have in Mods folder that I barely use) because...well...I could uninstall something important by mistake and regret it afterwards. After that I have to inspect what item is missing or what could I use to replace it.

Reseting the lot (Nraas MasterController) doesn't affect the progress, but resets all current actions, teleports sims to safe place at the front door usually (no matter if they are at home or somewhere else) and refreshes the routing data in the lot.