1) Internal Short in the Thing my board was installed in. The PC Serial Port supplied an AC Path to ground. : Dead Laptop

2) Internal Short in the Thing my board was installed in had a short through a transistor insulator. The Inverting Buck Boost Supply Provided -24 volt to the PC also making use of the chassis ground. : Dead PC.

3) Connecting to a CPU that was using a 16 volt mid rail as "Ground" with out dis-connect All the other test equipment. : Dead PC

So I guess I am saying check for shorts. Measure the voltages to the chassis. Unless there is some thing connected to his board he left out. The MAX232 can not do more than +/-10VDC even broken. The PC can handle +/-12. His Supply is 9VAC It may not be able to generate enough voltage to blow the PC if connected directly. So I get 2 theories. 1) A short in the AC Supply. 2) His DC regulator is putting out too much voltage, causing the MAX232 to double it to a high enough value to damage the PC receiver. This would leave the transmitter undamaged.

The baud rate change is a "red herring" (just a distraction) It can not damage the PC at normal baud rates.