Same as Petr, I'm using fedora 26
Everything was fine yesterday.
I tried downgrading java to previous version (from java-1.8.0-openjdk.x86_64 1:1.8.0.144-7.b01.fc26 to java-1.8.0-openjdk.x86_64 1:1.8.0.131-7.b12.fc26), removing my .java and .minecraft folder and re-downloading Minecraft without success, still stuck on the same error message "Error: Could not find or load main class net.minecraft.client.main.Main"
Using launcher 1.6.82-j and Minecraft 1.12.2
Just in case, here's a link to the most recent post on this issue https://bugs.mojang.com/browse/MC-121292