if it worked before upgrade it doesn't mean that it should work after transition to 5.5 especially for this release which was in some cases completely redesigned ...
I know its a little bit painful process but it could be your issue....
If you decided to bypass account changes try to reregister vCenter components between each other, your self-signed certificates
could play also some role with this issue, for detailed steps follow this KB:
VMware KB: Re-pointing and re-registering VMware vCenter Server 5.1 / 5.5 and components
Regards,
P.