I can't fight with any of my pets in it, even though they are happy, bloated and have full health.

A friend of mine has the same problem.

I wondered, is that extended to many people? And what is the reason for it?

It's just a glitch of sorts. I get this problem all the time even though my battle pet is always bloated and very happy, just try to battle again later. it could just be a hiccup from the new things they updated on it.

Okay, thank you very much :) The problem is that it isn't working for two days now - it already happened to me previously but it solved itself 5 minutes later.