I already patched GNU screen so I made it past the 'loginName too long', but now I can connect, nothing shows up. I installed the driver as instructed here. Directly after I downloaded the driver from the above link it did work, the next time it showed all scrambled letters and now it doesn't show anything. I tried minicom as well, but that didn't work either, it only showed scrambled characters (not even alphabetic, sometimes). How to get either of them to work? I am sure that Raspian boots up normally, as I can still SSH into the pi.
@11684's problem was solved by my comment, as you can see here.
Just posting this as answer so he can mark it as answer and get this officially answered.
If you are seeing scrambled characters, it may just be the baud rate is wrong
Have a look at your
/boot/cmdline.txt to see what baud rate is specified there. Usually it's 115200
Other settings to check are
8 bit/no parity
Make sure you have the ground wire connected between the PL2303 and the RPi