This log message indicates that the OCSP certificate validation is not working properly. You should fix this in the OCSP settings in the Onboard CA (so it will add a reachable OCSP URL to the client certificates), or for now create a custom [EAP-TLS] Authentication Method that has an overridden OCSP URL; or switch off OCSP validation. This screenshot shows the OCSP override as an example:
In here you can also disable Verify Certificate, to check that it is indeed the OCSP. The OCSP URL can be found in the CA list.