No fix, no apology for having a broken product yet again, not even any communication.
MoviePlan.Plex, whose account is marked as a developer account, replied a few times yesterday. They are working on a fix.
Just wanted to add to the thread, same-same, Nvidia Shield Pro auto updated,   Plex and PlexAmp broken. PMS is installed on usb stick etc etc…
Patiently waiting for a fix. Thank goodness that this is a first world problem.
So, I completely nuked Plex & Plex Media Server on my Shield Pro 2019 with Adopted Storage USB SSD.
Delete cache, delete data, uninstall apps.
Reinstall app (using the test PMS version that was posted yesterday), delete cache, delete data. Then open Plex to trigger the initial setup.
After setting it up completely from scratch, I’m still having all of the same problems I outlined a couple posts above, here.
And also, if I reboot the Shield, PMS does not start back up. However, if I reinstall the same version of PMS on top of the existing install, it starts back up again.
Guess it’s back to Netflix for a while… lmao so sad
-Updated to Shield Experience v9.0
-PMS wouldn’t start no matter what I did incl device permissions etc.
-Contacted Nvidia, they spent half an hour diagnosing it and pushed out HOTFIX to my device
-HOTFIX didn’t fix the issue of being able to change PMS data storage location to external USB drive.
-nVidia support advised me they are going to continue looking into this.
If you haven’t lost your Plex data on external I’d recommend waiting till a fix is released.
This did not work for me. Installed it, rebooted and still no PMS poping up in my Plex Client
I’ve found that PMS only starts immediately after installing the APK, and does not restart on a reboot. Make sure you have your file access permissions for PMS set to “Always Allow” (or “Allow” if that’s not an option, as it isn’t for me). Then reinstall the the APK again on top of the existing install.
Assuming you get access to the server, you may need to scan your libraries again…
This worked for me
Server is not running… After re-install, clear cache and data!
That worked for me as well - Thx @audiomixer !
i have 2 16 tb hds atached and after the update my plex on shield p[ro no longer works. says no internet conection when there is. netflis, disny plus prime video all still work, its just plex.
I guess I am one of the fortunate ones. I just upgraded to Shield Experience 9. I did not try to open up Shield and went to the Google Play Store and ran updates which updated the Plex Server. As soon as that updated I went into settings and the Plex Server storage permissions already looked correct and then I checked the Plex app storage permissions and I did set that to allow. I rebooted and all of my library are on 5 different 5TB USB drives connected to a USB hub and all of them are working without any issues.
Tried it . . . no go for me. 
Ultimately it will likely come down to a proper fix by the developers.
no luck unfortunately.
According the the Plex Announcements page, there is a new version of PMS:
However it doesn’t look like it’s available yet on the Google Play Store…
Not even sure this would address the issue.
that’s a lower build number than the plex server apk artifact listed above unfortunately.
I tried to go into the Plex settings and under “PLEX MEDIA SERVER” there was nothing there. So I went into the Play store and tried to uninstall the Plex Media Server. Then rebooted Sheild, went into Plex, settings, and set up my directories for all of my media from scratch again. It’s scanning all of my folders which will take forever because I have so much but I think it will be successful. The only thing I worry about is that all of my music tags (genre etc) will be gone. I put so much work into that stuff! 
After updating to Android 11, my Plex server wouldn’t play movies from my external drive, said they were unavailable. Library wouldn’t update and I was unable to edit file location. I went into Plex media server settings, from the shield setting app on the Home Screen, under system apps. Under permissions, I changed it to allow access all the time. I opened Plex and it updated my movies and is working properly now. If that helps anyone, I don’t know.
Interesting… I tried the same thing and am still unable to change file location in Plex from “Internal” to my external drive.