[cobalt-users] Strange CMU error - migration to RAQ 550

[Pizza Box manager <cobalt-list@xxxxxxxxxxxxx>]

Hello to all,
We try to use the CMU to upgrade to our recent RAQ550;
Things are worked pretty much okay with a first few sites, however on the 
most important one (300 pop accounts),
the restauration process totally fails with 1 message per account :

Creating user: (userA here)
ERROR: Cannot find site group for user: (userA here) fqdn: (full domain 
name here)
Creating user: (userB here)
ERROR: Cannot find site group for user: (userB here) fqdn: (full domain 
name here)

etc...
I have only tried the operation once, because between backing up, 
transfering and trying to restaure,
this makes the client websites and all email accounts be suspended for 
nearly 1 1/2 hour...
I guess there no way to work on this without suspending the accounts ?
(Or could we leave the domain name on the old server, backup data and 
reopen the accounts on the new server, and then
synchronize one last time ? with Rsync ? so that we get all changes occured 
in the meantime ?)
All my backup files are still here on the old and new servers.
Terribly preoccupying situation for us...
Does any body have a clue ?

Best regards,