Yeah, something went seriously wrong with the license apps. I've had horrid issues with hserver and sesinetd after the update.
Long story short, i've had exactly the same issue. What's more -- even going back to 20.5 builds resulted in the same crash-after-splash-screen, even though it was all running fine the same day.
What worked for me:
- Full uninstall of Houdini and License Server through Houdini Launcher
- Removal of /Library/Preferences/sesi (entire folder)
- Machine restart (for all the daemons to load/unload)
However, I doubt you actually have to perform all of these. The issue seems to be very silly. I went ahead and ran Houdini through macOS terminal, only to notice a bunch of permission issues reported for various files (license included). So... I just ran Houdini from terminal, but as superuser.
If all this sounds completely unfamiliar to you, pretty much just run Terminal from (Applications/Utilities/Terminal.app) and, assuming you have the 21.0.440 installed in default location, just copy-paste and run this:
sudo /Applications/Houdini/Houdini21.0.440/Houdini\ FX\ 21.0.440.app/Contents/MacOS/houdinifx
You will be asked for password, but the input is hidden -- you won't see any visual feedback (**** when typing) just type the password and click enter.