BUG: Missing Sims and empty UI when arriving in vacation town

Problem
This problem exists in two variations

1. My Sim disappears when arriving at the vacation destination. The UI is loaded without the Sim and therefore it's empty
2. I travel with a group of people but when arriving at the vacation destination one or more of the Sims are missing.

Cause

1. According to Twallan there is a problem with the EC Code. A error that occurs when exporting the Sims to the vacation town is ignored resulting in the missing Sim. A second cause of this is that if you have a teen traveling they must have their homework skill at least at 5. If that is not the case the system is trying to adjust this. Sometimes it fails, causing the problem. 
2. Sims that travel together must know each other. Sometimes that is not the case for Sims in the same household if they have never interacted. Make sure they know each other before sending them on vacation.

It's possible that there are more causes to this problem

Solution
Load a older backup and try again. Make sure to save before going on vacation so that you don't have to go to far back if you get the error. It is possible that the error occurs reproducible every time though.

If you can reproduce the error every time you try after loading a backup, try switching active families, save the game, then switch back and go traveling again (Credit to davebignell)

Before going traveling, make sure to empty out any items from your family inventory, especially gifts you have received (Celebrity and otherwise). (Credit to DarkAmaranth1966)

Steps you can try if you don't have a backup or the error occurs reproducible
The following methods might be able to save your town. I did not try those myself, trying is at your own risk.

Solution von QueenBeewannabee in the official US forum

1. Make a copy of your savegame folder
2. Go to town edit mode
3. Click on any random family in town and select them as your active household. 
4. Your active family should now be playable. If you look on the vacation tab that lets you know how much time is left at your destination that means you are good to go at the next step 
5. Click on phone, computer etc and "go home" 
6. Once you arrive home, your original sim family (the one that dissapeared) should be back in place.
7. Your random sim family from vacation are also now added as members to the family. 
8. Simply kick out additional sim family and you will now have your original family safe and intact.

So far, all my inventory items have been safe however I have always lost my simoleons.

Solution by zerobandwidth in the official US forum (others have posted similar issues and a big thank you goes to Jaghar7, who research a lot on this issue)

1. Make a copy of your savegame folder
2. In the COPY directory delete the .nhd file(s) corresponding to the area(s) where you're experiencing problems.
3. Start Sims 3 and open your COPY save.
4. Choose one member of your household to travel to the location.
5. Wait for the travel operation to crash Sims 3, taking you back to the desktop.
6. Start Sims 3 again, and open your COPY save again. 
7. As a test, send one person to the troubled location again.
   EXPECT: At this point, the travel process will succeed, and your one sim will land in the remote location, and your Sims 3 dashboard will be intact. The game has recreated the location's neighborhood data. The location is now playable again, but...
   THE PRICE YOU PAY FOR PROGRESS: Any edits you've made to the area are now gone, and (this needs verification) all dungeon areas have been reset. After all, you just deleted the neighborhood. All adventure progress is saved with the Sim, so that won't be lost.
8. Bring your test sim back home.
9. If your test sim makes it home, and there are no other observed errors, SAVE YOUR GAME, preferably with another new name.
   EXPECT: The new save directory will have a newly-created .nhd file for the location that you tested. 
10. Rinse and repeat for additional locations if desired. 

Mods that solves the problem
The StoryProgression Mod von Twallan solves both issues and prevents them from happening in the first place. To solve the home work issue, you will need Twallan's MasterController to manually set the homework skill to 5 (Advanced\Skill level). There might be other causes that are not solved by the mods.

Further Reading

• BUG: Missing Sim