I was having the same problem on my server. As hinted in the description, it's not a Minecraft issue, but rather a JDK issue. Pinpointed it to this JDK issue:https://bugs.openjdk.java.net/browse/JDK-8154757
Apparently, this has been fixed in JDK 8u101 which adds the root certificates for LetsEncrypt for (see https://www.oracle.com/java/technologies/javase/8u101-relnotes.html)but the launcher still ships 8u51.
I was having the same problem on my server. As hinted in the description, it's not a Minecraft issue, but rather a JDK issue. Pinpointed it to this JDK issue:
https://bugs.openjdk.java.net/browse/JDK-8154757
Apparently, this has been fixed in JDK 8u101 which adds the root certificates for LetsEncrypt for (see https://www.oracle.com/java/technologies/javase/8u101-relnotes.html)but the launcher still ships 8u51.