This is caused by the 'smart nav' menu option on the box page. When you hover your mouse over a Pokémon, it will load a brief overview of it in the menu panel to save you having to click on each Pokémon to see more info about it.
If you're moving your mouse back and forth across a lot of Pokémon very quickly, it might not be able to keep up and that error message will pop up. If it does, just click OK and hover over the Pokémon again.
@Pikachu3828 you are right, it didn't happened when I put my mouse over one pokemon, but did when my mouse went over loads and after clicking the smart nav, I noticed that it loaded the pokemon, didn't realize that as it didn't open automatically lol
It does it to me even when I don't select smart nav or even mouse over any pokemon... usually when I click a pokemon or change boxes before the page fully loads (and after switching my sort options the rotating loading thing from the smart nav box shows under my pokemons)... I think it's caused by the smart nav not fully loading, because after I get that error and try to use smart nav in the box I switched to it doesn't work... (does smart nav load all the pokemon in your box when it first loads?)
and if it's when you are clicking a pokemon, it should just keep going to the pokemons page if you don't click anything (at least it does for me... that pops up, but it still continues doing what I told it to)