TYPO3 v. 10.4 URI Builder Produces Sometimes Faulty Language Links in the Cache

Question

After clicking a while through the web site, the pages in the cache change suddenly to faulty pages. One part of the links then are written with a-tags that don't have a href attributes. The other part are proper links to the correct page but to another language. It happens mostly to the default language (German). Some links are appended with &L=3 or &L=4 after the last forward slash. The id of the language changes. After clearing the cache the site works well until the next sudden break-down of the links. When the cache stays switched off everything works fine.

base: 'https://metu.de'
baseVariants: {  }
errorHandling: {  }
languages:
  -
    title: Deutsch
    enabled: true
    base: /de/
    typo3Language: default
    locale: de_DE.UTF8
    iso-639-1: de
    websiteTitle: ''
    navigationTitle: ''
    hreflang: ''
    direction: ''
    flag: de
    languageId: '0'
  -
    title: English
    enabled: true
    base: /en/
    typo3Language: en
    locale: en_US.UTF8
    iso-639-1: en
    websiteTitle: ''
    navigationTitle: ''
    hreflang: ''
    direction: ''
    fallbackType: strict
    fallbacks: ''
    flag: us
    languageId: '1'
  -
    title: French
    enabled: true
    base: /fr/
    typo3Language: fr
    locale: fr_FR.UTF8
    iso-639-1: fr
    websiteTitle: ''
    navigationTitle: ''
    hreflang: ''
    direction: ''
    fallbackType: strict
    fallbacks: ''
    flag: fr
    languageId: '2'
  -
    title: Italian
    enabled: true
    base: /it/
    typo3Language: it
    locale: it_IT.UTF8
    iso-639-1: it
    websiteTitle: ''
    navigationTitle: ''
    hreflang: ''
    direction: ''
    fallbackType: strict
    fallbacks: ''
    flag: it
    languageId: '4'
  -
    title: Russian
    enabled: true
    base: /ru/
    typo3Language: ru
    locale: ru_RU.UTF8
    iso-639-1: ru
    websiteTitle: ''
    navigationTitle: ''
    hreflang: ''
    direction: ''
    fallbackType: strict
    fallbacks: ''
    flag: ru
    languageId: '5'
  -
    title: Spanish
    enabled: true
    base: /es/
    typo3Language: es
    locale: es_ES.UTF8
    iso-639-1: es
    websiteTitle: ''
    navigationTitle: ''
    hreflang: ''
    direction: ''
    fallbackType: strict
    fallbacks: ''
    flag: es
    languageId: '3'
rootPageId: 1
routes: {  }
websiteTitle: Metu

What might be the problem? It isn't possible to switch off the URI builder. How can I track down the error?