Cause was in lua. There was a case which could result in minbudget and maxbudget being set to less than zero. If you want to fix it by script, would have to find these stations and set their budgets to zero or greater, but the cause would still be there.
@Murry and Coops, at the moment, the budgets of your stations are set when you explicitly transfer money to or from their accounts. They try to maintain the amount of money that you send them, with max budget set to some amount greater, and min at some amount less than the amount that you set. They consider money surplus when they accrue funds half again as much as their max budget, and send you enough money to drop their funds down to max budget.
