Installer log:
JVM info: Oracle Corporation - 1.8.0_51 - 25.51-b03
java.net.preferIPv4Stack=true
Current Time: 16/08/2023 22:14:20
Host: files.minecraftforge.net [51.79.83.165]
Host: maven.minecraftforge.net [51.79.83.165]
Host: libraries.minecraft.net [13.107.226.45, 13.107.253.45]
Host: launchermeta.mojang.com [13.107.253.45, 13.107.226.45]
Host: piston-meta.mojang.com [13.107.226.45, 13.107.253.45]
Host: authserver.mojang.com [18.66.122.115, 18.66.122.99, 18.66.122.3, 18.66.122.46]
Found java version 1.8.0_51
Added Lets Encrypt root certificates as additional trust
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Saw similar forums, but no solution problem resolved itself there. Also saw a very complicated solution that requires some knowledge of java that I don't have.