Email links are not yet pinned to a trusted host,

The Custom Base URL isn't really the setting you want here, and it's almost
certainly what broke your site's display.

Custom Base URL changes how Grav builds every URL on the site, not just the
ones in emails. Two things commonly go wrong with it:

1. If Grav is installed in a subfolder (for example
   https://www.meinedomain.de/grav), the Custom Base URL must include that
   folder: https://www.meinedomain.de/grav. If you leave the folder off, Grav
   drops it from every CSS, JS and image URL and the site renders unstyled.

2. It must match exactly how you reach the site in the browser, including www
   vs non-www and http vs https. If you browse https://meinedomain.de but pin
   the base to https://www.meinedomain.de, things can break in the same way.

The good news: you don't need Custom Base URL to clear that warning. The
warning only asks that email links are pinned to a host you trust, and the
Login plugin has its own setting for exactly that, which touches nothing else
on the site.

So do this instead:

1. Set Custom Base URL back to empty (Configuration > System > Advanced, or
   custom_base_url: '' in user/config/system.yaml). Your site should display
   correctly again.

2. Go to Plugins > Login > Security > Site Host and enter your site address,
   for example https://www.meinedomain.de

   Or in user/config/plugins/login.yaml:

   site_host: 'https://www.meinedomain.de'

3. Clear the cache (bin/grav clear) and reload the admin. The warning should
   be gone.

That's option 2 on the docs page, and for most sites it's the better one to
reach for.

If you'd still like to get Custom Base URL working, tell me the exact address
you type into the browser to reach the site (including any folder after the
domain), and what "no longer displays correctly" looked like: missing styles,
broken images, or a blank page? That will tell us straight away which of the
two problems above you hit.