I just checked and it is /dev/ttyUSB0. If I remove the board, the port disappears.
Besides, I just tried writing some rubbish for the serial port in the avrdude command line and I still get the error. So obviously it never comes so far as to address the port.
To me it looks like a configuration problem, I just can't figure out how and why it complains about the parallel port. I use the same config file that works on my desktop pc.
Also, I tried with my Uno instead. Now the port is /dev/ttyACM0, but the error is unchanged.
Incidently, I already found that page before asking here, but I did not fully understanding the implications.
I realized that the config files contained a lot of pre-configured board types of which many of them was of type parallel, having this line:
type = par;
Now, what was certainly not obvious to me was that avrdude failed when reading these config lines because of the missing system parallel port, even though I did not specify any of the programmers in question!
I think this behaviour is downright silly, but well, I replaced all occurences of par; with serbb; and then it worked.
I have even found that I don't need the reset line which is necessary on my desktop pc.
Thanks for your reply, kg4wsv; even though your suggestion did not help in itself, you made me look once again at the problem with fresh eyes