MIDI Clock & Triton

Hi,
 
I have a Triton Rack and I'm trying to sync the arp patterns via X1's MIDI clock. Unfortunately (as you guessed) it doesn't work. I did set the Triton's MIDI Clock to "External" and checked X1's "Transmit Midi Start/Continue/Stop/Clock" and "Transmit MTC".
 
What else could prevent the Triton from running the arpreggio? As soon as I set the Triton to "Internal", the sound will play with the arp (but NOT synced to X1), and with "External" on, the sound plays without the arp.
 
I'm using the MIDI I/O on my MOTU 828mk2.
 
Any input appreciated!