This is because of expiry of certificates used to register SVM with NSX. There can be multiple reasons for this :
Case 1: The NTP server(s) configured on NSX Manager and ESXi(s) are different. If the NTP servers on the host and ESX do not match, it is possible that the certificate validity start time is out of sync with the NSX Application Platform NTP settings, and hence when such a certificate is registered, NAPP trust manager throws an error that the certificate is invalid.
Case 2: Due to various reasons (eg. CPU or memory resources insufficient, etc) the SVM might not be able to power-on on the host. If the time required to resolve this issue is more than 30 minutes, the certificate created for SVM-NSX communication might expire and there will be a communication error when the SVM finally powers on.
Case 3: On a scale cluster with more than 50-60 hosts, depending on the network and storage latency, it might take more than an hour for the SVM to be deployed and powered-on on a few hosts. This causes authentication errors in communication between SVM and NSX.