My fighter is in the Lodge, he's fed, happy and at maximum health, but when I go to challenge opponents I get

"Oh no! None of your pets are eligible for battle. You might want to make sure that they're well fed and happy!"

Is he already in a battle? Check via your stats page?

How embarrassing! Totally forgot that, had finished off an opponent but didn't hit the game over tab

No problem, I do that a lot myself, actually haha. :)

Was trying out a new weapon last night and just totally forgot that poor Sky was in electric limbo