ext_conf_template.txt -- pathAdditionalConfiguration = '' (important for TYPO3 12)
I suggest this value to be deleted or at least be set to '' as default. If set, it triggers some weird checkings in ConfigurationService.php. For our case /typo3conf/ was not writetable and /LFEditor/ was set as symlink to get the overriden data out of the way of automated deployment. Anyway, the checking for /typo3conf/ seems to be unnecessary and it is triggered by this constant value set.