Hello future reader 
I’d like to add one more option to the ones above when facing “PKIX path building failed” issues. Again, these issues are often caused by a company proxy intercepting SSL certificates in order to monitor encrypted traffic.
Possibly, the proxy’s certificate has already been preconfigured in Windows by your IT. In that case you can simply instruct KNIME to use the Windows trust store. To do so, add these two lines at the end of the knime.ini file which is located in the KNIME installation folder (likely in Program Files).
-Djavax.net.ssl.trustStore=NONE
-Djavax.net.ssl.trustStoreType=WINDOWS-ROOT
Afterwards, you’ll need to restart KNIME.
Kind regards
Marvin