No, it doesn't work (Pixel 2XL, Android 11).
Now uninstall the app and try again.
It shouldn't work until you check the CheckBox.
On Android 10+ you can no longer ask for WRITE permission, because this is no longer availably.
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" android:maxSdkVersion="28" />
Need this permission, how solid file manager
android:name="android.permission.MANAGE_EXTERNAL_STORAGE"
Thanks Anke,
I noticed that and tried to ask read permission, but this only work when defaultFileScope set to legacy.
Where is the documents for file scope block?
Worked add manifest) 
Please, do NOT post apk files in the community at all.
Yes, I mentioned that several times, e.g. here.
Whenever you need READ / WRITE permissions for devices with Android 4.4 up (API 19+ / KitKat) you must set defaultFileScope = Legacy to get READ / WRITE permissions declared in the Manifest.
The same with Niotron.
But on Android 10 there is no problem with deleting files from /Download and /Documents or after uninstalling / reinstalling the app. These issues are only present on Android 11.
thank you for your tests...
did this work on the AI Test server or only in Niotron?
toFileName cannot have slash in starting
(taken from the Niotron thread)
What is the block Material_Textbox1.Text block about? What is its content? An empty string?
this will not help (except your app is a file manager app or similar), see also 저장소 기기에 있는 모든 파일 관리 | Android 개발자 | Android Developers
the app's usage of the permission must fall within permitted uses, and must be directly tied to the core functionality of the app. If your app includes a use case that's similar to the following examples, it's likely to be allowed to request the MANAGE_EXTERNAL_STORAGE permission:
File managers Backup and restore apps Anti-virus apps Document management apps On-device file search Disk and file encryption Device-to-device data migration
see also Use of All files access (MANAGE_EXTERNAL_STORAGE) permission - Play Console Help
Taifun
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First I got the same result with Niotron as with AI2 (app crashed).
Then I got a an APK after this issue was fixed by from Krish Jha (Niotron team).
Did you test this APK?
Btw, what do you think about this?
Have they given any insight into how they addressed the crash? An APK is of somewhat limited value here.
it's not a link,what you posted is the apk. 
The APK is hosted on the Niotron server.
So again: No reason to remove it.
Yes, I asked for the Java code or an APK, here:
@Anke
It is a direct download link, so @SteveJG is correct. Please change the link to a landing page for the link, not a direct download link.
I believe this would be correct:
No, because we need the solution here in MIT App Inventor...
@Anke you forgot to answer this question
Why do the file names have to be changed? I would asume, that we can overwrite any existing file?
Taifun