The game doesn't start: try the legacy branch on the "betas" tab.

[Ignis]

The current update is not working a few players.

The reason is because the CPU does not support AVX https://en.wikipedia.org/wiki/Advanced_Vector_Extensions

The symptom is a game crashing directly at the start of the game, with a log file (%userprofile%\AppData\LocalLow\Unknown Worlds\Subnautica Below Zero\Player.log) containing these lines:

========== OUTPUTTING STACK TRACE ==================

0x00007FFD4FF01574 (lib_burst_generated) Ordinal0
0x00007FFD4FF03E1E (lib_burst_generated) burst.initialize

...
 
========== END OF STACKTRACE ===========

We know the previous update was working fine, the minimal requirement on the store page asks for AMD Ryzen 3, which is a 2017 CPU, and not much older than that.

The legacy branch contains the new features from the current patch (Seatruck dock, Custom game mode).

To switch to this branch, right click on the game, properties, betas tab.
Choose the legacy branch, and close the window.

Steam will download this branch, and you should be able to launch the game again.

Note: we had reports about players who couldn't launch the legacy branch even after the fix. Because their CPU doesn't support SSE 4.1 either (2007 CPU...).

For this last case, delete or rename lib_burst_generated.dll in: Steam\SteamApps\common\SubnauticaZero\SubnauticaZero_Data\Plugins\x86_64