How to get around "invalid directory path" with UTL_FILE (ORA-29280)?

I feel rather stupid, but I cannot, cannot figure out how to open a file with UTL_FILE.
I have "create ANY Directory" rights, I created a directory ... pointing it to a local Win10 folder ("C:\TEMP" or "H:\TEMP").

I granted rights to said Dir-object. AND yes, the folder DOES exist on both my C-Drive and my H-Drive in this example.

I have a standard example ...

I create "fhandle UTL_File.File_Type;" ... and I have years of experience writing instrumented PL/SQL.

This code has a statement ...

fhandle := utl_file.fopen('C:\TEMP','TESTFILE.TXT','A', 1024);

... no matter WHAT I put in there as the location (c:\temp, c:\temp\, C:\TEMP, C:\TEMP\ ... or TEMP_LOCAL dir-object-name) ...