@Baukhol
There are things that you do in the game that will cause a lot to show as modded in the Gallery even though no mods are present in your mods folder. So, check the lot to see if you have any paintings crafted on an easel, photos taken with a camera, any crafted items from the fabricator, candles made with the candle station, knitted items like hanging plants. Pretty much anything a sim can make in the game. Paintings and photos are the usual things.
If you don't have any of those things on the lot, then try saving to your Library. After doing that. Exit the game. Then rename your The Sims 4 folder to something else. Start the game and create a new one. A new The Sims 4 folder will be created. Once you've made the sim and placed them on a lot, save and quit. Then copy your Tray folder from the old folder to the new one. Start the game using Load and select the save you just made. Then place the lot from your Library. Enter the lot in build mode from the map and make a small change (like place a deco item somewhere). Then save the lot to your Library again. Check to see if it still shows as modded. It shouldn't but it might so you'll have to do it all again. Sometimes it might take several times before the lot shows as mod free.
Tbh, sometimes you just can't get a lot to show as mod free doing this. I've had that happen. So, then you can try moving a sim into the lot and then move them right back out and keep all the furniture. Then go back to the lot in build mode from the map and save it again and see if it will show as mod free in your Library. If it does, then go back in and redecorate and refurnish the lot. Then try saving again to see if the lot still shows as mod free. It should.
Hope this helps.
