Access to the path ... is denied

Hello

I work on a program tu download FTP files to the server (using dotnet)
I use this instruction : SIOFile.WriteAllBytes(filename, MemoryStream.GetBuffer());
Which gives me the error message :
"A call to the System.IO.File.WriteAllBytes failed with the message :
Access to the path ... is denied"

The path is on F:\...\...\... and as a user I can create, read & delete Files in this folder
Even if I run NAV as Admin I get the error
The security on the folder is set to Total control for my user
In my program I successfully used dotnet to upload files
In my program I also successfully create copy and delete files (with nav instructions)

Can you please help me to find other hints to get this work ?
Thanks