Galifre,
Usually this message will come up if you're battling a bit too quickly. If you hit the 'Fight' button before the player's stats/picture has shown up, sometimes this will happen. I bet if you get that message again and just refresh that player's page, you'll be able to fight them.
It's a nice thought that Jolt would be keeping tabs, but I think the reality is that they have too few resources to worry about this type of monitoring in addition to other game bugs and expansions.