It really seems to be a power problem. When the WiFi adapter is directly plugged into the rpi it does not connect at all (although the power between TP1/TP2 is stable at 5,02V-5,04V; I am using a 1A power supply). The USB hub is only lend, so I will buy one from the "RPi VerifiedPeripherals" Wiki page. I will report back next week if this helps.
The PI's USB ports are unsuitable to power a WiFi adapter!
A wifi adapter isn't able to work with 4.4 Volt, (or less) which is about what remains after the USB polyfuses (designed for simple keyboards and mice) when drawing the kinds of currents any WiFi adapter will draw.
Simple USB keyboards (and mice) are designed to work with 4.4 Volt, and will also draw low enough amounts of current to make sure that the voltage after the polyfuses will stay above 4.4 Volt.
A wifi adapter draws too much current for the USB ports of a PI
A wifi adapter cannot work on the kind of voltages an USB mouse works with, which is all the PI will give the USB device.
The PI is only designed to work with these 4.4 Volt devices, try "anything funny" and you are using the PI for something it wasn't designed for?
Blame the USB polyfuses for that, (which are chosen for 100mA max 4.4 Volt devices) and the fact that the PI was designed for use with a 5V 0.7A power supply, which only has 200mA assigned to the USB ports..
Either use the non compatible device behind a powered USB hub that doesn't draw power from the PI
(the 5V from the PI shouldn't be connected to the 5V of the hub, as some cheap hubs do, ignoring USB specifications, so the hub can also be used without a PSU) or bypass the USB polyfuses, (that is, solder a wire over them) a procedure nicknamed the "PI-pass" in these fora.
Search for "polyfuse" for more threads about this issue.