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Hola Buenos días, yo uso la versión de Unity 2019.4.14f11; primero usaba una versión de Unity 2019.2.0f1, no tuve problemas hasta que cambie de posición mis proyectos, y me aparecía el error Copying assembly from 'Temp/Assembly-CSharp.dll' to 'Library/ScriptAssemblies/Assembly-CSharp.dll' failed, busque como solucionarlo primero reimporte los Sprites y Scripts, y me función pero cada cierto momento me aparecía el error y me molestaba, luego seguí buscando en Internet mi error y la otra opción era que tenia un malware y examine mi equipo con malwarebytes y no tenia ningún malware luego use Eset para examinar mi equipo y no tenia nada, luego pensé que era por la versión de Unity que tenia y me actualice a la vercion 2019.2.0f1 y no tuve problemas hasta que programe un Script y me apareció nuevamente el problema, reimporte de nuevo pero no funciono y no puedo trabajar con ese error por que no se va.
Me encanta programar en unity y es mi programa favorito para crear juegos pero me encontré con este problema y quisiera que me pudieran ayudar
Gracias

Turn off your antivirus software,Not realizing it caused me a lot of trouble.
关掉你的杀毒软件。

Unity 2019.1.7.f1
I think I may have found the solution, although I am not sure what it was, but this is what I did:
I went to Window > Package Manager
I installed Multiplayer HLAPI
installed XR Legacy Input Helpers (The one that caused me this error)
Removed XR Legacy Input Helpers
installed XR Legacy Input Helpers (Yes, I did this twice)
Removed XR Legacy Input Helpers
I opened Android Studio and installed API 23 & 24, after it was complete, I went back to Unity
I reinstalled XR Legacy Input Helpers
And the error was gone. Hope it helps someone.

Actual for 2019.3.10f1

Happened here in 2019.3... after upgrading the Post Processing effects package.

Removing that multiplayer package solved it, I don't know why. It’s unbelievable how unstable Unity became, lots of hours lost each update... Seems a random problem with the package manager.

This issue continues to persist and none of the suggested fixes work for us.

I had the same error,
Update the multiplayer package worked for me on 2019.2.5

Same error , i get these using 2018.3:
Copying assembly from 'Temp/Obi.dll' to 'Library/ScriptAssemblies/Obi.dll' failed
Copying assembly from 'Temp/Unity.Postprocessing.Runtime.dll' to 'Library/ScriptAssemblies/Unity.Postprocessing.Runtime.dll' failed
Copying assembly from 'Temp/Unity.TextMeshPro.dll' to 'Library/ScriptAssemblies/Unity.TextMeshPro.dll' failed

I'm going to comment this here, because I _THINK_ I found the culprit/fix. At least, it's still working for me...

I went into `Preferences->External Tools` and increased `Maximum JVM Heap Size, MBytes` from 4096 to 6000.

I'll report back if it's not actually a fix, and hopefully others will try it and report too.