FYI, If you are wanting backups of just the VM's, not application aware backups, then you don't need snapcenter.
I have snapcenter 4.2 deployed along with the databroker for application aware backups. I deployed the databroker, uninstalled the old VSC plug-in from SnapCenter and then added the databroker without issues. When you added the databroker to snapcenter what host type did you select? You need to make sure you select the type of vSphere and then put in the IP address and credentials you defined. Are you able to see the snapcenter plug-in in vCenter?
I have the exact same problem. This is coming from an upgrade of SnapCenter 4.1.1. Registering the Data broker I only get the 500 error.
The other thing I've noticed is that in the Data Broker settings for vCenter, it doesn't accept nested domains, ex... microsoft.contoso.com which out company uses. Is there a way around this? I can't ever get it to properly register with vCenter.
I have no idea of nested domain, but remembered Snapcenter asked me to type an IP address of Data Broker, not a hostname. How about ping to Data Broker from SnapCenter server and checking open port of firewall?