Downloading new board definitions in the Arduino IDE’s board manager may fail due to certificates missing from the JRE certificate chain. This may for example happen with the ESP32 board, where the host dl.espressif.com is using a Let’s Encrypt certificate.
The Let’s Encrypt root certificates can be added to the JRE keystore using the following instructions:
https://drissamri.be/blog/2017/02/22/trusting-lets-encrypt-java/
Most interestingly, the JRE keystore seems to be password protected , the password being… changeit 😈