I wouldn't be too worried about putting a high AC or DC voltage into the Aux input. They're high impedance and no significant current will flow.
But possibly the amplifier uses a bridge-tied load and has its outputs sitting above 0V (at half the power amplifier's supply rail voltage). If that's the case, if you connect either of the speaker wires to earth, that will damage the amplifier, or at least cause a protection circuit to activate.
Removing the short should clear the problem, but since it doesn't, I guess something has been permanently damaged in the output stage.
What is the manufacturer and model number? You, or someone here, can Google it and try to find a service manual or schematic diagram, to confirm that it's a BTL output with a DC offset, and see what components might have been damaged.
For example it may just be a small resistor, which lost its smoke quickly and isn't visibly damaged.
If it isn't, I can't think of any likely explanation offhand. Although if a service manual is available, it will have some explanation for the "f61" error code... even if it's just "take it to a repair shop".