TYPO3 Core

Enable synchronization for header field in tt_content:

typo3/sysext/frontend/Configuration/TCA/tt_content.php:

'header' => [
   ...
   'behaviour' => [
      'allowLanguageSynchronization' => true,
   ],
],

• Create a page
• Create two page localizations
• Add default content element 'Regular text element' or similar in default language
• Use page module Language view
• Translate content element to first locaziled page:
  "translate button" in column -> modal -> translate -> default
  The first translation is created successfully
• Translate content element of second localized page, but select first localized page as translation source!
  "translate button" in column -> modal -> translate -> first localizaton (german for me) -> do -> modal does not close.

Inspecting network of last modal shows a fatal:

Uncaught TYPO3 Exception
Argument 1 passed to TYPO3\CMS\Core\DataHandling\Localization\State::update() must be of the type array, string given, called in typo3/sysext/core/Classes/DataHandling/Localization/DataMapItem.php on line 320

This only happens if a content element has a different l10n_source that default lang in the modal.