I cannot give my assurance for the mini-PC you've got, but the following works well on my Nexus 6 running stock Android 6.0.1 with MX Player v1.8.3. 

Alright, so discard whatever actions you've setup and follow this simple one:

Code → Run Shell → 

 * Command: 

        am start -n com.mxtech.videoplayer.ad/.ActivityScreen -d "file://FILE" --user ID --activity-clear-task

  where, 

  * FILE refers to the location of your media file in Android. E.g. /sdcard/Movies/lolcat.mkv, /storage/emulated/0/Prime_time/after_dark.mkv and so on. 
  * ID refers to the [ID of the user account currently active](https://android.stackexchange.com/a/123209/96277). For primary user, it is always 0.
  * [--activity-clear-task](http://developer.android.com/reference/android/content/Intent.html#FLAG_ACTIVITY_CLEAR_TASK) is a flag which makes sure no multiple instances of the same activity are spawned. <br><br>

 Example command:

        am start -n com.mxtech.videoplayer.ad/.ActivityScreen -d "file:///sdcard/Movies/The egg of the King.mp4" --user 0 --activity-clear-task

 If MX Player is the default video player in your Android, for fun, you can replace `-n com.mxtech.videoplayer.ad/.ActivityScreen` with `-a android.intent.action.VIEW` in the said command.

 * tick Continue Task After Error

You can now setup a wait timer or anything that you intend to do after the execution of that action. In order to fire a new video, just copy everything from aforesaid action except of FILE which needs to be replaced with new file location. 

In case Tasker doesn't work out, try with [Automate](https://play.google.com/store/apps/details?id=com.llamalab.automate&hl=en). It has a neat block named [App Start](https://llamalab.com/automate/doc/block/activity_start.html) which is specifically meant to launch an activity of an app.

<sub>(Click image to enlarge it)</sub>

[<img src="https://i.sstatic.net/ajTXtl.png" width="210" height="350" alt="IMG: " title="">][1]

 [1]: https://i.sstatic.net/ajTXt.png