Blur effect not working with Adjustment layer

Hello all,

Quick question from a Adobe premiere 2020 beginner, please dont judge me to hard if the question is very obvious :).

I have a project consisting out of multiple video clips, in one of these video clips i have 2 persons that i want to blur (Gaussian blur with the Track selected Mask option). Normally i would add an Gaussian Blur Effect directly to the video clip itself, but for this clip (duration 20 seconds) 1 person is not visible anymore in this clip after 8 seconds en the other one after 12 seconds. 

So i would like one blur to stop after 8 seconds and the other one after 12 seconds, i think i need the Adjusment layer option for this, so far so good.. But when i add the new adjusment layer and apply the Gaussian Blur effect and i hit the "play"button for the Track Selected Mask it does not show any progress in the progress bar, if i cancel it i doesn't allow me to press the "play" button again because it's greyed out..

*When i add the Gaussian Blur to the clip itself it all functions fine.

What am i doing wrong?

My system is a MacBook Pro (13-inch, 2018)

Premiere version: 14.0.3 (Build 1)

I've have already searched the internet on:

Selected Mask tracking not working with Adjustment layer

How to solve blur problems in adjustment layers

etc. etc. But i won't give me the solution to my problem,

I already tried to clear the Cache of Premiere but no good luck 😞 (https://community.adobe.com/t5/premiere-pro/blurring-through-adjustment-layers-does-not-seem-to-preview-or-render-properly/m-p/10994912?page=1#M258372)

Greetings Kenny