after doing some research from threads a ex-dev guy discovered this change is responsible:
1.5.9 ( num2 = elementCopyAtIndex.Number - elementCopyAtIndex.WoundedNumber; )
num2 = Math.Max(0, Math.Min(num2, (party.MobileParty.PaymentLimit - party.MobileParty.TotalWage) / (partyWageModel.GetCharacterWage(elementCopyAtIndex.Character.Tier + 1) - partyWageModel.GetCharacterWage(elementCopyAtIndex.Character.Tier))));
'In a nutshell, the TroopTree has been (?inadvertently?) capped at Tier 7. Exceeding that will trigger the error. Again, not sure it was on purpose, because without this code bug, exceeding Tier 7 would not crash this function.'
Anyway I just wanted to bring this to attention and keep up the great work on the game; hopefully in a future patch this can be looked at as its a limitation to a thing that made so much more possible before.