Confirm transformer fault vs markings / spec
13V sounds reasonable for open cicuit. The transformer output will be 9V RMS at 220V input and a resistive load with 800mA RMS on both secondary coils. I don't know the configuration, maybe the secondary coils are in series. Then you would have 18V at 800mA.
I guess there's a short somewhere on the secondary side. You can measure the resistance of the secondary input when the transformer isn't connected. Maybe a rectifier diode is gone. Definitely the resistance of the circuit should be way more than few Ohms.