adminhtml – Admin redirect loop after 2.4 upgrade

we are working on updating 2.3.5 to 2.4.2-p1
Frontend loads ok, but admin backend creates a redirect loop admin/admin/auth/login/key/…. to admin/admin/index/index/key/… and admin/admin/auth/login/key/ again.
DB and config are roughly the same as on 2.3.
When requesting with a valid login cookie created by mage 2.3, I can enter admin backend ok, so this looks like general deployment is ok.
I’m a bit lost understanding what the system is supposed to do. I diffed the module-backend with no obvious change in that area.
As far as I can see on 2.3, there is some magic that actually lets vendor/magento/module-backend/Controller/Adminhtml/Auth/Login.php render the content when index/index is requested and no user is logged in. We seem to get a redirect to auth/login instead, which then fails due to Login.php:54-56, which detects the wrong URL and sends the user back to index/index.

nginx,config.php, php, varnish unchanged from mage 2.3

Any help or hint where to look highly appreciated.