1. Failed to import package with error: Couldn't decompress package UnityEditor.Web.JSProxyMgr:DoTasks()
Maybe 7z.exe under <Program Files>\Unity\Editor\Data\Tools is infected with virus and deleted by security software(which is installed later).
2. Could not start compilationWin32Exception: ApplicationName='"...Unity/Editor/Data/Mono\bin\mono.exe"', ...
In my case, the mono.exe get deleted.
You can reinstall Unity or copy a good 7z.exe and mono.exe from your friends' computer.