loick,
If I understand your issue correctly when you reboot the appliance the boot sequence is stopped at "Starting synchronization service" Below is a link to our document on this issue. The simple version is login as admin and run "clear syncdata." If you are unable to log into the box using ssh than your only resolution is an OS Restore using the disk image .iso file.
I have included links to both process below.
Title: 'Brightmail Gateway stops Boot sequence at starting LDAPsync daemon'
Document ID: 2009111307211054
> Web URL:
http://service1.symantec.com/SUPPORT/ent-gate.nsf/docid/2009111307211054?Open&seg=ent
Title: 'Restoring Symantec Mail Security 8100 Series, 8200 Series, 8300 series or Brightmail Gateway, appliances from the installation CD'
Document ID: 2005040414130363
> Web URL:
http://service1.symantec.com/SUPPORT/ent-gate.nsf/docid/2005040414130363?Open&seg=ent
Thank You,
John