Left it run for the night, come back the following morning, U3 isn't running. Go and have a look, seems that the power brick burnt out. After getting in contact with the store, they tell me that the frequency was set too high and that's why it was running slow and overheated the power block. Since I see no way to set the frequency in Minera, he recommended me to use MinePeon, or directly from the PC.
Finding your U3 zombied is par for the course, it's how these things operate. They might run solid for a couple days, they might zombie out many times in a single day, they are not very stable machines.
The factory power bricks are notoriously crap, as are the USB cables they come with. Use a good usb cable on a powered usb hub and see if that helps.
If there is no way to set the freq in your setup, then how could it be "too high"? It would be at stock 225 without any additional parameters. And it's bumping the voltage up that burns out the power bricks, not freq.