Happenned just after the upgrade of some wall parts. Thought it was a temporary bug, but the balance remais negative as I´m earning some gold from caravans and mine. Can´t make any attack until have some positive gold again.
Peace treaty finished. Couldn't make an attack before. Received an attack and I don't know how but the attacker loot some gold from my negative balance.
Hold your device at arm's length. Now bring it toward your forehead rapidly until you can go no further. Extend your device at arm's length again. Repeat.
It won't fix the problem but after a while it'll feel strangely therapeutic and you won't care ....