CODE: SELECT ALL
Ships Tax: gc 18,400
This tax has been added to your nation debts.
You have an outstanding debt of gc 83,900 (Details)
83,900+18,400= 102,400
It seems much more likely that you missed another ship payment (as it is set to manual as you said yourself) than it is that this is a bug of some sort.
You keep saying you had no debt; if anything, it seems the bug is that your screen said you had no national debt... Not that you actually didn't have any debt.
Why not set it to automatic so it isn't an issue unless you don't have enough?