I see. I stopped using full control all that often, because i think it is very annoying for the opposing side and in this case, i admit that i've forgotten to do so. At least, I find it very annoying, if my opponent is in full control all the time, even when i quite literally "know", he doesn't hold a response in his hand. But that is a different matter. I'll try using stops, or full control next time, but i believe there is no specific stop i can "tell"arena to look for, in that case.

Well, if you could cast it later that turn, then you should have been able to cast it then, too.

My guess then is, that it being an upkeep trigger, Arena may require a stop in the opponent's upkeep in order to not automatically pass through it for you.

@Rezzahan thank you for clarification. I do understand the priorisation, but there wasn't any opportunity to respond to it at all. Only after he cast his next spell, i was able to respond with the flash mechanic, not to a trigger of the heart, nor after the creatures (Jin Gitaxias, the other one i can't remember) entered. Anyways, i might be missing something and if so, would you mind to elaborate?

The creatures are searched for and put onto the battlefield all during the resolution of Dedense of the Heart's trigger. You do not get priority in between. You can respond to the Defense's trigger, not knowing what will be searched for, or you can respond after the creatures have entered the battlefield, all appropriate triggers have been put on the stack, and the active player has passed priority to you.