404 error in admin panel when logging in with deep link

[TomOne]

When I’m logged out of the admin panel and try to access it with a deep link (e.g. https://my-grav-site.com/admin/config/system instead of https://my-grav-site.com/admin), a 404 error appears after the login.

Reproducible example:

1. Make sure you’re logged out of the admin panel.
2. Open https://my-grav-site.com/admin/config/system in the browser.
3. log in
4. After the login, the admin panel redirects to /admin/system (stripping out /config/), leading to a 404 error.

This happens with other URLs too, e.g. https://my-grav-site.com/admin/pages/home, redirecting to /admin/home (stripping out /pages/). It seems to always remove the path segment that comes after /admin and makes a 303 redirect to that URL, potentially producing a 404.

My environment where this issue appears: fresh Grav 1.4.5 installation with Admin 1.8.2 and Login 2.7.0 on Debian 9, PHP 7.2 and Apache 2.4

[rhukster]

sorted for next release.