I believe this was one of the lnown bugs in the German Beta, please try following the below instructions
After selecting Settings> Apps> Show system applications in the upper right corner,
1. Select "Media Storage"> Storage> Delete Data
2. Select "Sec Media Storage"> Storage> Clear Data
My phone doesn't have the option to clear data just cache. And the error keeps showing Every few seconds. Unable to revert back to android pie because play store won't work due to the error so I can't install smart switch
Make sure you choose to show system apps and clear the storage on the Sec Media Storage as well as media storage
I am glad that this solved your problem!
If there are any "known issues" we will usually share them either in "Beta notice" or in the notice for joining the Beta, so it is a good idea to check those first!