It's caused by using the Arduino IDE with OneDrive. You need to install the hourly build from:http://www.arduino.cc/en/Main/Software
Yes. You need to use it. The bug with OneDrive is fixed in the hourly build of the Arduino IDE. If you keep using Arduino IDE 1.8.4 then of course you're going to continue having the error.
Unzip the downloaded file. You will now have a folder named arduino-nightly.You can move the arduino-nightly folder to any location you find convenient.Run the program arduino.exe that you find inside the arduino-nightly folder. You might find it useful to create a shortcut of that program and put in in your Start menu or desktop.
Not sure how to answer that.
The file you download from there website.
The one you need to run everything.