The command line args specified in the README file in this repository are for an older version of GTNH. This is also where the wiki points the user to find how to setup a new install with MultiMC.

Attempting to run the client with those args results in a number of confusing errors followed by a crash.

-Dfile.encoding=UTF-8 -Djava.system.class.loader=com.gtnewhorizons.retrofuturabootstrap.RfbSystemClassLoader -Djava.security.manager=allow --add-opens java.base/jdk.internal.loader=ALL-UNNAMED --add-opens java.base/java.net=ALL-UNNAMED --add-opens java.base/java.nio=ALL-UNNAMED --add-opens java.base/java.io=ALL-UNNAMED --add-opens java.base/java.lang=ALL-UNNAMED --add-opens java.base/java.lang.reflect=ALL-UNNAMED --add-opens java.base/java.text=ALL-UNNAMED --add-opens java.base/java.util=ALL-UNNAMED --add-opens java.base/jdk.internal.reflect=ALL-UNNAMED --add-opens java.base/sun.nio.ch=ALL-UNNAMED --add-opens jdk.naming.dns/com.sun.jndi.dns=ALL-UNNAMED,java.naming --add-opens java.desktop/sun.awt=ALL-UNNAMED --add-opens java.desktop/sun.awt.image=ALL-UNNAMED --add-opens jdk.dynalink/jdk.dynalink.beans=ALL-UNNAMED --add-opens java.sql.rowset/javax.sql.rowset.serial=ALL-UNNAMED

vs

--illegal-access=warn -Djava.security.manager=allow -Dfile.encoding=UTF-8 --add-opens java.base/jdk.internal.loader=ALL-UNNAMED --add-opens java.base/java.net=ALL-UNNAMED --add-opens java.base/java.nio=ALL-UNNAMED --add-opens java.base/java.io=ALL-UNNAMED --add-opens java.base/java.lang=ALL-UNNAMED --add-opens java.base/java.lang.reflect=ALL-UNNAMED --add-opens java.base/java.text=ALL-UNNAMED --add-opens java.base/java.util=ALL-UNNAMED --add-opens java.base/jdk.internal.reflect=ALL-UNNAMED --add-opens java.base/sun.nio.ch=ALL-UNNAMED --add-opens jdk.naming.dns/com.sun.jndi.dns=ALL-UNNAMED,java.naming --add-opens java.desktop/sun.awt.image=ALL-UNNAMED --add-modules jdk.dynalink --add-opens jdk.dynalink/jdk.dynalink.beans=ALL-UNNAMED --add-modules java.sql.rowset --add-opens java.sql.rowset/javax.sql.rowset.serial=ALL-UNNAMED
per GTNewHorizons/GT-New-Horizons-Modpack#15714 (comment)