[Defender] MASK LOGIN AREA

Hi Support,
I am trying to mask the WP login area.
Note that the wp core is in a subdir, application.
When i update the settings i get the following message: "Masking is currently active at https://clickster.nl/application/dashboard"
But when i resolve https://clickster.nl/application/dashboard i get a 404.
When i resolve https://clickster.nl/dashboard i get a login form. But if you login (you dont have to enter any credentials) i get a 404 again.
Do you have any ideas how to fix this?
Thanks in avdvance!
Best regards,

Makke