Same issue, just a little more infomation to add to the thread. I have replaced the default certificates on my vCSA 5.5.0.5101 build 1398493, so when I attempt to re-regeister the inventoryservice it doesn't complete successfully.
Successfully loaded VC certificate
Cannot read VC private key file /etc/vmware-vpx/ssl/rui.key
java.lang.NullPointerException
at com.vmware.vim.dataservices.vcregtool.RegisterVC.loadVcPrivateKey(RegisterVC.java:478)
at com.vmware.vim.dataservices.vcregtool.RegisterVC.loadVcProviderInfo(RegisterVC.java:300)
at com.vmware.vim.dataservices.vcregtool.RegisterVC.register(RegisterVC.java:203)
at com.vmware.vim.dataservices.vcregtool.RegisterVC.doRegistration(RegisterVC.java:1253)
at com.vmware.vim.dataservices.vcregtool.RegisterVC.main(RegisterVC.java:1332)
Failed to perform register action
com.vmware.vim.dataservices.vcregtool.exception.RegistrationException
at com.vmware.vim.dataservices.vcregtool.RegisterVC.loadVcPrivateKey(RegisterVC.java:483)
at com.vmware.vim.dataservices.vcregtool.RegisterVC.loadVcProviderInfo(RegisterVC.java:300)
at com.vmware.vim.dataservices.vcregtool.RegisterVC.register(RegisterVC.java:203)
at com.vmware.vim.dataservices.vcregtool.RegisterVC.doRegistration(RegisterVC.java:1253)
at com.vmware.vim.dataservices.vcregtool.RegisterVC.main(RegisterVC.java:1332)