TYPO3 Core

can't confirm this by testing

when i change typo3/index.php line 145 redirectToURL to alt_main.php this will do the trick for me.

Had same problem. Fix does work.

I am experiencing the same problem after an upgrade from 4.0.9 to 4.1.7 en from there (after running the update wizard) to 4.2.1.

interfaces = 'backend' results in the old interface and 'backend_old' in the new, 4.2 interface
These values do appear in the loginform in the hidden 'interface' field

workaround would be: use interfaces = backend_old if you want the new one and vice versa

I can confirm this. Only the login will bring you to alt_main,php any other invocation of a URL with typo3/ will always redirect to backend.php.

Seems that the BE interface has to be stored in the session for later use.

After the old BE was removed in #22701:

Resolved no change required.