Ardour cannot export with focusrite

I am testing a Focusrite 2i2 and I cannot export my session.
The exporting dialog hangs forever.

However if, in jack2, I change to another card, all is working perfectly.

What is strange is that the other parameters of the two jack configurations are exactly the same!

I am using archlinux, jack2-git (compiled without dbus) and the last version of ardour. Kernel is the last realtime kernel.

Moreover, I have found that I cannot use focusrite as input and another card as output, because of periodic xruns (every 150 seconds aprox.)

If I use the same card for in/out xruns disappears and I can achieve around 2,6 ms latency at 88.2 kHz (ardour is showing 1,5)

This extremely unlikely to be due to the Focusrite, since export takes place without any interaction with the audio interface.

I suggest you run Ardour 4.4 with the ALSA backend and see what happens.