There are two viable options for your reference that are tailored to your problem. The first option is to downgrade your Shield Experience 9, as this issue is caused by unknown bugs in its upgrade. The second option is to update Shield Experience to 9.1.1, which was released a few months ago.
The downgrade is particularly troublesome because you need to back up all your files and factory reset your Plex. An unlocked bootloader is also indispensable. Hence, I recommend you upgrade your Shield Experience to 9.1.1 instead.
As Experience 9.1.1 claims, this minor upgrade fixes four common yet annoying problems:
1. Resolves an issue where the TV remote would not control SHIELD 30 seconds after waking SHIELD.
2. Resolves issues with CEC volume control not functioning properly on AV receivers or soundbars.
3. Fixes bug with files not detected properly on connected removable storage.
4. Resolves an issue with the SHIELD TV app not launching on select Samsung Galaxy phones.
Do not hesitate to upgrade to Experience 9.1.1 and check if you still receive the notification that reads "The operation can't be completed because you don't have permission to access some of the items."
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