WP Admin Redirect Not Working all it sudden

Hi,

We enabled the WP-Admin URL redirect and it works for a long time, but all it sudden it s not working.

When I try login to WP admin

https://mywebsite.com/wp123

then it redirects to https://mywebsite.com/wp-admin/
and saying “Not available” on the screen.

But If I do the same using the incognito windows to log in, I get the enable cookie warning first login attempt, but then if I try to log in again, it works.

Could someone time what is the problem? is that my browser session problem?

Thanks!