Beta launcher cannot launch the game because the Java main class cannot be found

When trying to start the game with latest beta launcher it will fail to do so with a "Game crashed" error. (stable 2.1.5964 works fine) In the launcher_log.txt it looks like it can't find the main class even though it's clearly inside the version jar file? (And changing to a different version/making it redownload doesn't solve it)

More information like the system environment and tested java/Minecraft versions in the attached launcher_log.txt.

Possibly related to MCL-12098 although my launcher doesn't crash in Windows itself and the other issue is with an older launcher version.