SCOM agent not starting after installation

Today I was upgrading a number of SCOM agents from SP1 to R2 version (using split monitoring at first).
After the installation the agents refused to start.
After a manual try on the machines to start the service I got the following error:
Windows could not start the System Center Management on Local Computer. For more information, review the System Event Log. If this is a non-Microsoft service, contact the service vendor, and refer to service-specific error code -2147467259.
Alright, so I check the System event log. Nothing to see. So I check the OpsMgr event log and I see it starting and saying it knows of two management groups and will start to try to contact them etc etc and suddenly followed by informationals that it is shutting down again.
Alright, so I found and checked for the WindowsAccountLockDownSD key in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\HealthService\Parameters\Management Groups\mymanagementgroup .
Sure enough it was not there for one of the two management groups.
So I picked up the value from another machine and tried to merge it with this one.
After merging it the service would not start. Great. :-/
So I removed the SCOM agent completely and discovered the machine again and installed the agent. Checked for the existance of that registry key and it was not there. So I merged the value again for the WindowsAccountLockDownSD and sure enough the service could start right away.