I've figured out what goes wrong, and just like to update the thread:
on the PCB, D3 and D5 of the controller were shorted during soldering, so when ever D3 is used, D5 gets messed. =) and D3 happens to be the TX pin for UART.
)
after some desoldering and cleaning up, now my keyboard is happily typing via BT =)
thanks for all the help!