Subject Firebird UDF Access configuration failing

For 15 years I have formatted the UDF Access parameter in Firebird.conf as below:

UdfAccess = Restrict UDF <UDF Folder>

where UDF Folder is the system folder for UDF files

example as below:

UdfAccess = Restrict UDF  C:\Program Files (x86)\Firebird\Firebird_2_1\UDF

Lately and only on Windows 10 machines. This has been failing.

It has failed on a Firebird 2.1 and a Firebird 2.5.8 installation.

To Fix it, the firebird.conf gets changed to "UdfAccess = Restrict UDF"

Then It works. By that I mean the UDF access is working again.

Does anyone know why?