OData calls in SAPUI5 (locally on Tomcat): HTTP request failed (500)

Hi!

I've got a problem with OData calls in SAPUI5 applications running locally on a Tomcat server.

I configured the proxy in my WEB-INF/web.xml:

----WEB-INF/web.xml--------

  <servlet>

  <servlet-name>SimpleProxyServlet</servlet-name>

  <servlet-class>com.sap.ui5.proxy.SimpleProxyServlet</servlet-class>

  </servlet>

  <servlet-mapping>

  <servlet-name>SimpleProxyServlet</servlet-name>

  <url-pattern>/proxy/*</url-pattern>

  </servlet-mapping>

  <context-param>

   <param-name>com.sap.ui5.proxy.REMOTE_LOCATION</param-name>

   <param-value>https://******.wdf.sap.corp:*****</param-value> <!--commented out real name-->

  </context-param>

Now when I try to do OData calls with the serviceURl /appname/proxy/sap/opu/odata/sap/appname/ I get the error:

HTTP request failed500,Internal Server Error.

sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

Does anyone know what goes wrong?

Thanks and best regards

David