Interesting post over at Technodrone on a common issue experienced when you replace SSL certificates in vCenter.  

If you replace the SSL cert in vCenter, you might experience an issue where certain plugins (namely the Health Monitoring plugins) will fail to load after you load the new certs.

The short story is:

• When you create the rui.pfx file, you must use the password 'testpassword' when you create the file.  No exceptions, this is apparently hardcoded into vCenter.