This will be very hard to reproduce, what I would suspect has happened is one of your requests finished first and someone got put on cooldown or a tier changed. Maybe you went to 0 points or something, It's hard to tell and it's very hard to reproduce as it would require simulating exact attacks.
The attack logs don't show two simultaneous attacks which means that the protection code kicked in and prevented the attack from going through, I think for some reason it's just not displaying an error but I'm unsure currently which check it's falling over at.
I've added a bit of code which should show when an error happened, it should simply read: "Error: We're not sure what went wrong.".
Hopefully this should never be seen.