I've upgraded a 4.5.5 installation that had also acted as a domain controller using the zimbra_samba and zimbra_posixaccount extensions.
After the upgrade was completed then I encountered problems in loading the zimbra admin console.
It seems the problem lies in the zimbra_samba and zimbra_posixaccount.
I had them undeployed and installed the newer version that came with the 5.0.6 installation.
After that I can login into Zimbra Admin.
However I could no longer see all the Domain accounts I had created in the Zimbra Admin.
These accounts are still visible in LDAP though.