> The UDF/DLL is in located in the path described in the firebird.conf: UdfAccess = Restrict D:\Apps\Firebird\Firebird_2_5\UDF;

Try
UdfAccess = Full

Check OS permissions of dll.

> Yes, "myDLL.dll' is 32-bit, However my FB installation is 64-bit. I did compile the dll to the 64-bit target and copied into the
> UDF folder.

Perhaps your UDF is dependent on antoher dll ?

Is this necesary ?
uses
System.SysUtils,
System.Classes;

> function Modulo(var i, j: Integer): Integer; cdecl;

Btw. Firebird has built in function MOD() (since fb2.1)

Ivan