TYPO3 Core

Siteconfig.yaml with a second language config which is currently not used at the backend - so no translation exists, tries to get the 404 page in the selected language (which does not exists).

For example:
SiteConfig:

languages:
  -
    title: Deutsch
    enabled: true
    languageId: '0'
    base: /
    typo3Language: de
    locale: de_DE.UTF-8
    iso-639-1: de
    navigationTitle: Deutsch
    hreflang: de
    direction: ltr
    fallbackType: strict
    fallbacks: ''
    flag: de
  -
    title: English
    enabled: true
    languageId: '2'
    base: /en/
    typo3Language: default
    locale: en_US.UTF-8
    iso-639-1: en
    navigationTitle: English
    hreflang: en
    direction: ltr
    fallbackType: strict
    fallbacks: ''
    flag: en-us-gb

If the Backend has no translation and no language defined, then you try to get "domain.tld/en", then TYPO3 tries to get the "domain.tld/en/404" page over and over again - infinite loop (which does not exist).
Until the webserver crashes.