Exchange 2010 (SBS2011) , one user can't connect

Just upgraded one site to SBS2011 a month ago, all seem ok, but yesterday, one user (out of 30) can't connect to the Exchange, Outlook 2007 just loops with 'Trying to retrieve', in the end, I reinstalled Office, reset the LAN, then after 30 mins, all worked again.
This morning, same thing happend, this time, I checked the  users OWA, and I get this :

An error occurred and your request couldn't be completed. If the problem continues, contact your helpdesk with this HTTP Status code: 500.

Request
Url: https://remote.xxxxxxxx.co.uk:443/owa/lang.owa
User host address: 81.xxx.164.xxx
User: Paul Xxxx
EX Address: /o=First Organization/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=Paul Xxxx
SMTP Address: paul@xxxx.co.uk
OWA version: 14.1.218.13

Exception
Exception type: System.Web.HttpException
Exception message: Request timed out.

Call stack
No callstack available

So I know the problem is from the Exchange rather than local.
Looking at the EV, I can't see anything wrong.
All the other users seem to work fine, if I OWA to all other users, they all connect first time, I tried to restart the Exchange Form Based Authentication, it made the login screen slightly faster but same error for this user.

Any experts know what can cause this ? or know how to fix it ?
Many thanks