If the gold balance was negative, increasing the resources could had caused increase of the negative gold balance. The happiness was not affected by the problem, attempts to make vassals don't lead to happiness loss. Are you still experiencing similar problems?
but it would be good (it was quite bad) if they had informed about it earlier.
I agree with you, unfortunately, such situations can’t always be anticipated and this time it wasn't possible to send you a prior notice. I apologize for the inconvenience.