Inno Setup 6 cannot use DLL function with string parameters, while it works in Inno Setup 5

We are currently using Inno Setup version 5.5.3 to build an installer. I am planning to upgrade this version to 6.1.2.

We use Inno Setup for installation of our product. We ship a license code along with installer, and user enters the license in a field during installation. This license is validated using a custom DLL and a non negative result is returned by DLL for valid license. This process works fine in 5.5.3 and 5.6.1, but fails in version 6 (tested with 6.0.5 and 6.1.2).

Unfortunately there are no logs generated that point the exact issue.

This custom DLL is 32-bit and is built using C++ over 10 years ago. There are no plans to redo this part. Is there any way to use the same DLL and fix the problem? Thank you.

[Files]
Source: mydll.dll; Flags: dontcopy

// This function calls the external mydll which parses licenseCode and
// returns an integer
function getFlags( secret, licenseCode : String) : Integer;
external 'getFlags@files:mydll.dll cdecl';

function checkLicense(license : String) : Boolean;
var
  secret : String;
begin
  Result := False;  // default is Incorrect license
  
  license := Trim(license);
  secret := <secret>

  // This line calls the above getFlags function and expects to get an integer
  license_flags := getFlags( secret, license);
  
  if license_flags = -1 then begin
    if not suppressMsgBox then begin
      MsgBoxLog( ‘Incorrect License’)
    end;
    Exit;
  end;

  Result := True;
end;

// This is the c++ function
PS_EXPORT(int) getFlags( const char * secret, const char * license ) {

  // This function is returning -1 for Inno Setup 6
  // but works fine for Inno Setup 5
  if (strlen(license) == 0)
    return -1; 

  ...
}