Here's the feedback from Draytek UK Support:

The 3.6.3_RC1 is a beta firmware and since then the 3.6.3sb has been released.

In close consultation with our development team we have noticed unusual behaviour
if a user with a HTC One X phone tries to connect then it will cause the Router to reboot.

The firmware was attached to the email - can't see it on a download site yet.

I'll update the router when I'm back in the office (Tues 15th Jan 2013)

Interesting. I'll stick with my RC1 version for now until it's confirmed the new sb version works! thanks for the info.

I have a site with the exact same problem. It started at 8.45 yesterday morning, after that its been restart almost every 10 mins.

Looged case with Draytek and initial reply was to factory reset (as if I haven't done that already)

Anyway hopefully they will send this RC3 firmware to test, I've not been able to trace an HTC but there was a new access point put on the LAN Friday afternoon :?: :?:

UPDATE

We have found an HTC ONE that is causing the the 2850 to restart.
Helpfully we will get the patch from Draytek....

Got the new firmware and I can confirm it has sorted the issue with the HTC ONE

I'm having the same problem, waiting for a reply from Tech Support. Did they send you another firmware version? My 2850n is up-to date already