Error called "VideoPlayer cannot play" in 2018.1.1f1

Hi everyone!

I'm having a problem called "VideoPlayer cannot play" with only this one line and no other information in Unity 3d 2018.1.1f1. I'm making a 360 video player in Unity, and using videoplayer script on a sphere. When I upgraded from a 2017 version to 2018 ver, I'm constantly getting this error notification for like, 20 seconds per one notification for all the time, and I can't find a way to solve it.
Below are the libraries I'm using (LSL is another plugin
using System.Linq;
using System;
using System.IO;
using System.Collections;
using LSL;
using UnityEngine;
using UnityEngine.Video;

It's my first time asking questions in the forum so I'm not sure what other information I should provide (if anyone's willing to look at those), shall I post my codes, or any other information is required for a good question?

Thanks!