Users often encounter an exception when validating certificates which is poorly worded and confusing:
Due to phrases like "PKIX path building failed" and "unable to find valid certification path to requested target", users often believe this is a file path issue and that NiFi cannot locate the truststore. However, the issue is actually that no certificate validation "path" can be constructed – i.e. NiFi cannot find a chain between the certificate being validated and any trusted CA certificate that has signed it (or intermediaries).
This exception should be caught and a more explicative error message should be displayed, with suggestions for how to resolve this issue (usually importing the custom CA certificate or self-signed certificate into the truststore).