Ok so this just happened on my phone as well, so it is not the operating system causing the error, most likely. I created a screen recording of the game and the error. 

me too!

I'm having the same issue as the OP. Spider 20 or 21.

I noticed that it happens when 2 spiders drop (usually the last 2). And if you notice in my screenshot and that of the OPs, those spiders seem to have oddly large HP bars for some reason. I think it froze around the 2:40 mark but the timer just kept on rolling until I took my screenshot and killed the App.

Platform: Windows 10

Edit (to add team in case it's an animation issue): Ursala, Sicia, Cardiel, Hurndig, Miscreated Monster

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Yeah this is a major issue. I can't farm spider during the event/trournament. 

And here I thought I am going crazy, this happend to me 8 times today!

It happend both while manually fighting and auto battle, both on my phone and on bluestacks (win10), both 1x and 2x speed and low and high res...

So nothing i can do about it but it already fu**** me over quite hard on the spider event.

edit: Stage 15 - Team: 2xElhain, 1x Suwai Firstborn, 1x Fahrakin the Fat, 1x Gorgorab

I woke up this morning, and the timer suprised me!

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this is still happening..already lost over 200 energy to this stupid bug

Hello,  ally  attack  champions  are  causing this  to  happen.  

Even if Ally Attack is the cause it definitly has something to do with the spider dungeon, I've been running ally attack on pretty much all other content for weeks without any issues at all.

I can confirm that this is the case for me also. I have been farming gear and pots like crazy for 2 weeks but Spider is the only place this happens to me. I lost out on getting my hippo because of it :(