You have successfully followed importing an existing vCenter to create a workload domain into VCF Operations.
You proceed to add the vCenter in VCF Operations UI under Administration - Integrations - ADD Account and receive an 'internal server error' when validating the connection.
VCF Operations 9.0.1
This issue has been resolved in VCF Operations 9.0.2
There is no workaround so upgrade to version 9.0.2