TYPO3 Core

Had the same problem after updating a project from 8.7 to 9.5, that had a sysfolder structure with pid=0, that also held translated records .
The solution was to simply create a very limited site config for that sysfolder as well, that just holds the rootPageId and the languages it needs:

rootPageId: 684
base: 'https://www.dummy.com/'
languages:
    -   title: Default
        enabled: true
        languageId: '0'
        base: /
        typo3Language: en
        locale: en-INT
        iso-639-1: en
        navigationTitle: ''
        hreflang: en-INT
        direction: ''
        flag: eu
... more languages

makes little sense for a multi-domain instance with shared records.

10 years ago I would have seen this narrow-minded restriction as a great innovation; haha.. no even at that time not .. but hey; what can I expect from a system that uses the unit pixel for responsiveness :D

.loon