Common Errors

All

Could not create Java Virtual Machine

In 32bit OS Envrionment, you can't set MaximumRam(XMX) exceeding 1024

Error: could not open jvm.cfg

Reinstall java.
if your launcher use MJava, remove runtime directory(default: <Your Minecraft Path>/runtime) and download java through MJava again.

macOS

DockName

You have to set DockName and DockIcon in LaunchOption.
If DockName is empty, you can't click or access to minecraft window.

example:

 var launchOption = new MLaunchOption
 {
     MaximumRamMb = 1024,
     Session = session, 
     DockName = "Minecraft"
 };

On macOS Catalina, Minecraft works normally without above option. but some macOS version doesn't work well.

JRE

Old minecraft version doesn't support OpenJDK 11.

Linux

Use Java 8. Old minecraft version doesn't support OpenJDK 11.

sudo apt-get update
sudo apt-get install openjdk-8-jre

and

java -version

It should return

java version "1.8.0_~~~"