I figure that I can safely say now that the problems are due to the device and/or the rt2x00 driver. I have been using Ethernet only the last 48 hours, copied many many Gigabytes of data, and haven't got a single problem. Hence, currently I would discourage the usage of rt2800 based devices, and I will try to find another dongle, maybe one with the Realtek chip, since that seems to be the most widely used.