The first thing in the checklist: are the times on the fling appliance and vCenter server in sync?
If they are and you still get this, we might need to take a look at the corresponding error that is generated after trying to login in the web client logs (under /usr/lib/vmware-vsphere-ui/server/logs/vsphere_client_virgo.log).
After installing the ova and connecting to our VCenter 6.5 appliance, we cannot login. Getting error: "[500] An error occurred while processing the authentication response from the vCenter Single Sign-On server. Details: Empty SSO response string.
Back to login screen". Please advise what we need to do to be able to login?