Arduino Mega not connecting to Unity

[Former user]

My project has been working great for months now with no issues. All of the sudden the Arduino Mega stopped connecting to the Uduino Manager. I get the following 2 errors:

The serial port of [\.\COM4] is open in another application. Please close this application first (Arduino IDE ? ) or unplug/plug the device.

AND

UnityException: FindObjectsOfType can only be called from the main thread. Constructors and field initializers will be executed from the loading thread when loading a scene. Don't use this function in the constructor or field initializers, instead move initialization code to the Awake or Start function. Uduino.UduinoManager.get_Instance () (at Assets/Uduino/Scripts/UduinoManager.cs:125)

I have restarted my computer several times and it still does not connect. I have also tried using the Arduino IDE and it DOES talk to the Arduino with the serial interface like normal. I have made sure the Arduino IDE is closed down when running Unity.

Please help me as I am in a time-crunch trying to get this game project across the finish line and this is a catastrophic issue for me.

[Marc Teys]

Increasing the "discover delay" solves the issue for Arduino Mega.