Since of today (Sept. 4th 2019) the launcher does not launch the game anymore.
Instead when I press the play button it fails with a "com.google.gson.JsonSyntaxException: duplicate key: null" (see full log above)
Others seem to have the same problem: https://www.minecraftforum.net/forums/support/java-edition-support/2985294-minecraft-launcher-stuck-on-preparing
If I had to guess, I'd say it has something to do with some info files format being changed to accommodate for the release of obfuscation maps, but that just speculation at this point.
Thank you for your report!
However, this issue is Invalid.
This is a technical support issue; this site is for bug reports only. We do not have the resources to provide you with technical support.
Please follow the steps outlined in MCL-6550.
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki