This page outlines some specifics related to running BepInEx of various platforms and Unity versions.
Whenever you have problem starting up BepInEx, most commonly it's either because of a missing core file or a wrong entrypoint.

Common

Enable console

In many cases, it's suggested to enable the console. That way you'll see load progress and potential errors live.

Open BepInEx/config/BepInEx.cfg, locate and change the following settings accordingly:

[Logging.Console]

Enabled = true

Remove Managed folder and verify files

If you're upgrading from older version of BepInEx or different modding frameworks, there might be some incompatible DLLs installed into the game's Managed folder.

If the game is on Steam, go to <Game Folder>\<Game Name>_Data folder and delete Managed folder. Finally, go to Steam and verify game integrity.
This will cause Steam to redownload a clean copy of Managed folder.

If the game is not on Steam, you can try obtaining the clean Managed folder or reinstall the game altogether.

(Windows) Check the bitness of the game

Currently Windows builds of BepInEx ship separately for x64 and x86 games.
Because of that, make sure the version of BepInEx is for the correct architecture.

To do that, run the game and check it via Task Manager.
If you see (32 bit) after the game process name:

the game requires x86 build of BepInEx.

If you don't see such addition:

the game requires x64 build of BepInEx.

Extremely long paths with non-ASCII characters

Some versions of Mono bundled with Unity games cannot handle non-ASCII characters in paths or too long path names. Because of that, it's suggested that

• Your game executable path is not too long. Under 1024 will work on most systems, under 256 on all.
• Attempt to remove "exotic" characters from the game path. Make sure any of the game folders have only the following characters:
  • A-Z, a-z or numbers 0-9
  • Common punctuation (.:;,!"#%&()[]{}=?*'_-)

Unity 2017 and newer

Change the entrypoint

In some games, the default entrypoint is too early for BepInEx to load up properly. For that, try an alternative entrypoint:

Open BepInEx/config/BepInEx.cfg, locate and change the following settings accordingly:

[Preloader.Entrypoint]

Assembly = UnityEngine.CoreModule.dll

Type = MonoBehaviour

Method = .cctor

Harmony backend

On Unity versions 2017 and newer (especially 2018), Harmony and MonoMod.RuntimeDetour may error when trying to patch anything. Here's an example error message:

[Error  : Unity Log] NotImplementedException: Derived classes must implement it
Stack trace:
System.Reflection.Module.get_Assembly () (at <e1319b7195c343e79b385cd3aa43f5dc>:0)
MonoMod.Utils._DMDEmit.Generate (MonoMod.Utils.DynamicMethodDefinition dmd, System.Reflection.MethodBase _mb, System.Reflection.Emit.ILGenerator il) (at <041d70ff506f4f089a67adab0245e45d>:0)
MonoMod.Utils.DMDEmitMethodBuilderGenerator.GenerateMethodBuilder (MonoMod.Utils.DynamicMethodDefinition dmd, System.Reflection.Emit.TypeBuilder typeBuilder) (at <041d70ff506f4f089a67adab0245e45d>:0)
MonoMod.Utils.DMDEmitMethodBuilderGenerator._Generate (MonoMod.Utils.DynamicMethodDefinition dmd, System.Object context) (at <041d70ff506f4f089a67adab0245e45d>:0)
...

This is due to the System.Runtime.Emit implementation in the version of Mono that is bundled with the game being incomplete. This can be fixed by setting the Preloader.HarmonyBackend setting to cecil, as such:

[Preloader]

## Specifies which MonoMod backend to use for Harmony patches. Auto uses the best available backend.
## This setting should only be used for development purposes (e.g. debugging in dnSpy). Other code might override this setting.
# Setting type: MonoModBackend
# Default value: auto
# Acceptable values: auto, dynamicmethod, methodbuilder, cecil
HarmonyBackend = cecil

Unity 5 and older

Change the entrypoint

In some games, the default entrypoint is too early for BepInEx to load up properly. For that, try an alternative entrypoint:

Open BepInEx/config/BepInEx.cfg, locate and change the following settings accordingly:

[Preloader.Entrypoint]

Assembly = UnityEngine.dll

Type = MonoBehaviour

Method = .cctor

In some cases, the another option works better

[Preloader.Entrypoint]

Assembly = UnityEngine.dll

Type = Camera

Method = .cctor

Future versions of BepInEx should automate the process of setting an early enough entrypoint.

Unity 4 and older

Ensure core libraries are included

Some older Unity games strip away unused core libraries. Specifically, BepInEx requires the following two libraries to be bundled

• System.dll
• System.Core.dll

Make sure they are included in <Game Name>_Data/Managed folder of your game.
If not, you have to obtain such libraries yourself at the moment.

1. Head to Unity download archive
2. Find an old version of Unity. 5.0.0 is suggested as a good middle ground
3. Download its Unity Editor and install it
4. Go to <unity-install-dir>\Editor\Data\PlaybackEngines\windowsstandalonesupport\Variations\win32_development_mono where <unity-install-dir> is the directory where you installed Unity to
5. In the folder, locate System.Core.dll (should be in Data\Managed) and copy it to your game's Managed folder
6. Try running the game again. BepInEx should now launch

Rename winhttp.dll to version.dll

While winhttp.dll proxy works best on more platforms (especially older versions of Wine on Linux), older Unity games might not work properly with it.

Try renaming winhttp.dll that comes with BepInEx to version.dll and run the game.