Login panel not displayed

I’m testing on Drupal 8.37 the free Drupal Theme – Custom Theme
it worked fine so far. but now i don’t get to the login panel or window.

can somebody tell me, how can I access as an admin directly from the browser the back-end without having a login panel

Thank you for help.

Drupal version: 

Source: https://www.drupal.org/taxonomy/term/3/feed