thanks for your response,
I got the same screen the first time I switched it on too. But, like you said, it disappeared within a few seconds and worked normally.
The only thing I can think that I may have done wrong was to switch off the mains supply before switching off the device. I've checked the power supply, it seems alright. Nothing else on the board seems fried, and the green power LED lights up. In case of a firmware glitch, is there any way of re programming it to fix the problem?
I bought it from Local Makers in Amsterdam.