I just started up a project and immediately got the error. It also persists in newly created projects.
I've tried deleting/Library, reinstalling Unity, reinstalling the Hub, updating Python(saw that on macOS it has to do something with it, I'm on Win11), fixing registry errors, renaming folders in the path to not contain spaces, and probably a couple of other things I can't remember now.
Full error on a newly created Project:
> Internal build system error. Backend exited with code 2.
STDOUT:
[ 0/410 0s] WriteText Library/Bee/artifacts/1900b0aE.dag/UnityEditor.UI.AdditionalFile.txt
[ 0/410 0s] WriteText Library/Bee/artifacts/1900b0aE.dag/UnityEngine.TestRunner.AdditionalFile.txt
[ 0/410 0s] WriteText Library/Bee/artifacts/1900b0aE.dag/UnityEditor.TestRunner.AdditionalFile.txt
[ 3/410 0s] WriteText Library/Bee/artifacts/1900b0aE.dag/Unity.Services.Core.AdditionalFile.txt
[ 4/410 0s] WriteText Library/Bee/artifacts/1900b0aE.dag/UnityEngine.UI.AdditionalFile.txt
[ 5/410 0s] WriteText Library/Bee/artifacts/1900b0aE.dag/UnityEngine.TestRunner.rsp
[ 6/410 0s] WriteText Library/Bee/artifacts/1900b0aE.dag/UnityEngine.UI.rsp
[ 7/410 0s] WriteText Library/Bee/artifacts/1900b0aE.dag/UnityEditor.TestRunner.rsp
[ 8/410 0s] WriteText Library/Bee/artifacts/1900b0aE.dag/Unity.Services.Core.rsp
[ 9/410 0s] WriteText Library/Bee/artifacts/1900b0aE.dag/UnityEditor.UI.rsp
STDERR:
tundra: error: Couldn't launch process with command line:
cmd.exe /c ""C:\Program Files\Unity\Hub\Editor\2021.3.9f1\Data\Tools\netcorerun\netcorerun.exe" "C:/Program Files/Unity/Hub/Editor/2021.3.9f1/Data/Tools/ScriptUpdater/ApiUpdater.MovedFromExtractor.exe" "Library\Bee\artifacts\mvdfrm\UnityEngine.RuntimeInitializeOnLoadManagerInitializerModule.dll_E51F0780B0D29005.mvfrm" "C:\Program Files\Unity\Hub\Editor\2021.3.9f1\Data\Managed\UnityEngine\UnityEngine.RuntimeInitializeOnLoadManagerInitializerModule.dll""
errno: 0 (No error) GetLastError: 2 (0x00000002): The system cannot find the file specified.