New version 15.1.0 is now available

[matija]

Can you please do a test for me?

In your ServerController:
- Cut all code in your OnNewSession event, including all comments and variables. Leave it only as:

procedure TIWServerController.IWServerControllerBaseNewSession(aSession: TIWApplication);
begin

end;

- Save the ServerController file again (the IDE should remove the empty OnNewSession event)
- Now, in Object Inspector, locate OnNewSession event and double click on it, creating a new empty event.
- Paste back the original code. The event should be linked again.

Rebuild your application and run it again. See if it fixes the issue

Still the same error!

Removed IWServerControllerBaseNewSession and again created.

procedure TIWServerController.IWServerControllerBaseNewSession(
  aSession: TIWApplication);
begin
    ASession.Data := TIWUserSession.Create(nil, ASession);
end;

Also tried as a new project with Result := TIWUserSession(WebApplication.Data).CheckUser(aUser, aPass); in TIWServerController.IWAutherEvent1Check