I am very interested by problem in mathematics which are difficult (go at least 10 years without a resolution, say) but which have a solution that is short and elementary.

In this video Launay give an example : https://www.youtube.com/watch?v=knyQyV4aXrA

Launay calls these problems "Pokémon problems": difficult to catch but once you catch them, they're in your pocket.

Question: What are other problems like this?