TYPO3 Core

Version: 11.5.4

It needed time to find the issue but found now a bug I can't really understand.

I got an error testing to send a mail:

Could not deliver mail
Please verify $GLOBALS['TYPO3_CONF_VARS']['MAIL'][*] settings are valid. Error message: Resource paths must start with "EXT:" 

All paths were correct, a tryout in another installation with same settings had no issue.
So I started to rebuild the whole system and tried step by step to send mails. It stopped when I set the Backend-Settings:

'backend' => [
    'backendFavicon' => 'medien/marketing/XXX_favicon.png',
    'backendLogo' => 'medien/marketing/XXX_logo-t3.png',
    'loginBackgroundImage' => 'EXT:site_xxx/Resources/Public/Images/XXX_Pressefoto-36_web.jpg',
    'loginFootnote' => '© 2021 by XXX',
    'loginHighlightColor' => '#d91882',
    *'loginLogo' => 'medien/marketing/XXX_logo-t3-login.png',*
    'loginLogoAlt' => 'Testsite',
]

It broke when I set the 'loginLogo' to an absolute path. It works with EXT: and //domain.io/path – that's why there was no error in the other system.