UnauthorizedAccessException when calling FileInfo.Length

I do get reports via Crashlytics, that some of the Users of my Unity app (roughly 0.5%) get an UnauthorizedAccessException when I call FileInfo.Length;

the interesting part of the stacktrace is:

Non-fatal Exception: java.lang.Exception
UnauthorizedAccessException : Access to the path '/storage/emulated/0/Android/data/com.myCompany.myGreatGame/files/assets/myAsset.asset' is denied.
System.IO.__Error.WinIOError (System.IO.__Error)
System.IO.FileInfo.get_Length (System.IO.FileInfo)

The corresponding File (it's a different file for every report) was written (or is currently written) by the same application (possibly many sessions earlier). The call happens in a backgroundthread and there might be some writing going on at the same time. But according to the .net doc this property should be pre-cached (see https://docs.microsoft.com/en-us/dotnet/api/system.io.fileinfo.length?view=netframework-2.0)

The whole code causing it is:

private static long DirSize(DirectoryInfo d) 
{
    long size = 0;
    FileInfo[] fileInfos = d.GetFiles();
    foreach (FileInfo fileInfo in fileInfos) 
    {      
        size += fileInfo.Length;
    }
    ...

Did anyone experience something similar and knows what might be causing it?

from UnauthorizedAccessException when calling FileInfo.Length