Hey I have done everything as you said, but I wasn't able to play by right clicking and selecting java8.
Only running from command line: "java -jar Runex.jar" it finally worked.
Just posting this for all other people still having issues.
Follow the guide (delete cache and client, then redownload .jar file).
Then download java jre 8, by using this two commands:
sudo apt update
sudo apt install openjdk-8-jre
After installation is complete you have to change default java version, use this command then type number corresponding java8 version:
sudo update-alternatives --config java
after that is done, just use command below to load Runex:
java -jar Runex.jar
Hope it helps somebody!
