Mhm, well now I kind of got it to work... Maybe I'm just using the wrong chipset or something. The Sanguino dudes use an Atmega644P but on my board there's the Atmega644-AU tqfp sitting, on the board itself is silkprinted "Sanguino.cc Atmega 644A".
So I went to the boards.txt of the (downloaded) Sanguino folder and simply changed the line "sanguino.build.mcu=644p" to "sanguino.build.mcu=644" because it was telling me something like "expecting signature 644p ...". So this error message doesn't show up anymore.
Instead, now the TX and RX leds of my ftdi boards flash around a little bit but after some seconds the big red led on the FlashBoard lights up constantly and the arduino IDE tells me "avrdude: stk500_recv(): programmer is not responding"...
Mhm, any clue?