Can confirm, getting this exact issue:
The new launcher update bricked my fabric loader launcher settings. I run Minecraft fabric with Java 15 instead of the pre-installed Java 8 JVM. This was all working fine until my launcher was updated. Now the launcher will either work with Fabric running Java 8, or Vanilla running Java 15, but not both. Also seeing two slightly different behaviors depending on how the launcher is configured:
If I set "keep the launcher open while games are running" and "open output log": The game hangs on "preparing..." then hard crashes
If I do not have the above settings on, the launcher just hard crashes
Output logs do not state anything useful from what I've seen; they don't even update on the crash.
Thanks for the fix!