I use wifi and 4g interchangably, so I don't think that's it...I
think it's a programming error judging from the code that showed
up. Also, the box is grayed out in the storage boxes section,
meaning I can't click on it and rescue my Pokémon.
Should be fixed now! (Note that your Box #0 is now your last box
due to this fix, as #0 is an invalid box ID caused by the storage
bug that was fixed a few days ago.)