According to IBM's documentation for their IBM Data Server Driver for JDBC and SQLJ, error code -4214 is an "Authorization failed" error -- and you know that much. But you can get more specific information about the problem if you call SQLException.getMessage. I think you may have done that already, but it's not entirely clear.
My semi-educated guess is that you haven't yet followed the configuration steps in IBM's driver documentation in the section "Configuring the Java Runtime Environment to Use SSL":
https://www.ibm.com/support/knowledgecenter/SSEPGG_11.5.0/com.ibm.db2.luw.apdv.java.doc/src/tpc/imjcc_t0054066.html
You'll have to apply some common sense knowledge to those instructions to adapt them for the OpenJDK, and this might not be an IBM supported combination depending on what/how you do it. If you can somehow use the IBMJSSE2 Provider, that'd be nice.
I think Mao and Mustaccio probably have the right ideas, and probably you're just missing the server certificate in your truststore. The instructions linked above walk you through how to configure that.