Now, since I posted an upgrade video of the Citrix Synchronizer yesterday, I wanted to share some knowledge that I have been troubleshooting for about an good 20 minutes now.
When booting up my XenClient 1.0 that was upgraded from RC2, I now get an error stating:

Error: ‘Could not get ID of the user’ when Logging on to a XenClient 1.0 Device Upgraded from RC2 which is Registered with a Synchronizer

Well, after doing some searching and digging on the Internet I came up without any good explanation to the problem…
So, I did a visit to the support pages for XenClient and there it was!

So, here is what you need to do:

Unregister your upgraded XenClient device from Synchronizer.

Using the steps below causes the following actions to occur:

Disassociates your downloaded virtual machines from the Synchronizer; that is, it converts them to local standalone virtual machines.
Unregisters your upgraded XenClient device.

At the end of the steps, you can access your local virtual machines and re-register your XenClient client device with a version 1.0 Synchronizer.  At which point you can choose to upload your virtual machines again.

Start your XenClient device and when the XenClient image on the boot-splash appears, press ESC.
Using the arrow keys, scroll down to the boot option XenClient Technical Support Option: console access. If you receive an error message stating “An error has occurred on this launch or the previous”, ignore it and let the boot process complete.
On the login prompt, use your root account and password that you set up during installation of XenClient.
Run the following command, and note the IP address. You need this later to log on to your XenClient device using SSH.
ifconfig brbridged
Reboot your XenClient device and allow the regular boot process to complete by running reboot.
Using an SSH client, SSH into your XenClient device using the IP address you noted in step 4.
Log on to the XenClient SSH session.
Copy and paste the following commands into your SSH session one by one, pressing Enter after each command.
SHUTDOWN_VMS=”managed_vms.each { |vm| vm.shutdown() rescue nil if vm.running? }”
UNMANAGE_VMS=”managed_vms.each { |vm| vm.unmanage() rescue nil }”
GET_VMS=”managed_vms, local_vms = xenmgr.get_all_vms.partition { |vm| vm.is_managed }”
sed -i “s/\($SHUTDOWN_VMS\)/\1\n$UNMANAGE_VMS\n$GET_VMS/” /usr/sbin/bed-deregister
bed-deregister –f
reboot

For the complete support article from Citrix Support click here