Performance of solar panel vs. DC power supply
A solar panels current rating is based on short circuit current (short the wires together) and it's voltage is open circuit.

When you attach a fan (an inductive load, relatively heavy load too), the voltage of the panel will drop and is probably less than 12V. However, because there is some current flowing through the fan coils, it will spin, albeit slowly.

Your solar panel CAN'T provide 800mA @ 17 V. It can provide up to 800 mA current and up to 17 V. Give or take.

The plug in power supply can deliver however many A @ 12 V. It will maintain 12 V. A solar panel won't.

You should use a battery and solar regulator to supply the power to the fan. As long as there is more power going into the battery than the fan is consuming over a long period, you should be ok. Remember the battery won't charge during the night so the panel needs to deliver enough power during the day that will be used in a 24 hour period. That assumes a sunny day every day too...