Restcall fails because JVM says it is "unable to find valid certification path to requested target"

When consuming webservices, sometimes the  JVM says it is "unable to find valid certification path to requested target". The publishing server, tieka.nl, has a valid certificate from authority 'Let'sEncrypt'. I have no way to retreive the certificate, can not download it from any browser and the provider does not give it to me. Also, how should I make the certificate known to the JVM? It is the second time that I come across this problem. Previous one was with consuming a service of BAG, Basisregistratie adressen en gebouwen, a governmental institute, so maximum trustworthy. Their certificate comes from authority Quovadis. So it is not likely that the problem is the certificate. Apparently JVM does not find the certificate. In other words, how can I solve this problem?   com.mendix.modules.microflowengine.MicroflowException: Error calling REST service    at PostalcodeHousenumber.ACT_LUTieka_Get_AddressData (CallRest : 'Call REST PDOK/suggest postcode+huisnummerRecieve address-id')    at PostalcodeHousenumber.SUB_Address_GetList (SubMicroflow : 'ACT_LUTieka_Get_AddressData')    at PostalcodeHousenumber.ACT_Call_Address_GetSingleAddress (SubMicroflow : 'SUB_Address_GetList') Advanced stacktrace:    at com.mendix.integration.actions.microflow.RestCallAction.execute(RestCallAction.scala:48) Caused by: com.mendix.systemwideinterfaces.MendixRuntimeException: java.security.PrivilegedActionException: javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target    at com.mendix.integration.util.PrivilegedUtil$.withPrivileged(PrivilegedUtil.scala:15)