That 3 pin connector top right is the fan, I suppose?
Well, in that case the serial port must be on jp2, as you suggested.
So I need to somehow figure it out by myself :S
Actually it's less difficult and risky as it sounds. You have a multimeter? First measure (with unpowered board) the resistance between ground (from the power connector) and the pins. You'll find at least 1 zero ohm. That's the ground. (duh). It is perfectly possible that you'll find 5 grounds, or something like that. Then power up the board, and measure the voltage between a ground pin, and the remaining pins. You'll probably find one or more 3.3V, and several zero V pins, maybe a 'in between', and maybe also a +5V or +12V pin.
All 'zero V' pins (not the ground pins), 'in between' and 3.3V pins are suspect of being RX or TX. But the 'zero V' pins are more suspect of being RX, while the others are more suspect of being TX.
All 5V and higher pins are suspect of being dangerous for your TTL port, so don't touch these.
Now you can start your serial terminal, set the baudrate to 115200-n-8-1. Short the RX and TX of your TTL cable, and type something. It should be echoed. If you break the short, the echoing should stop. Then the cable is OK.
Connect the ground to one of the ground pins, and probe with the TTL-RX line the TX suspects. Connect the pin, and power up the board. If you don't see anything in 10 seconds, repeat with next suspect, until you find the boot log. Then you've found the TX.
The RX is found basically the same, but it is possible you won't find it, as you'll need feedback of the NAS, and possibly the NAS never actually listens to it's RX.
BTW, you only need ground,RX and TX.
3.3V is only needed in the rare cases the TTL level port of your converter needs a reference voltage, or even power from that line.
About your nmap log, did you change ip address using NDU? It's not the default 'panic' address.