TYPO3 Core

It seems like the cause was solr trying to write something to the settings.php a fact that I had missed because on the development system I deleted caches and compared database with the cli. So on the dev system The settings.php did not get changed.

Now on the life system with logs and everything turned of for security reasons it just blew up the install tool with a 503 which is not a nice reaction for such a small cause.

I would suggest in such a case to just give a warning like "Your config/system/settings.php needs to be updated by Extension XY"...

I've found it happening because of missing EXTCONF values of installed extensions, so TYPO3 silently writes them when entering the maintenance module.

In my case, the scheduler was installed, we ran extension:setup, and we deployed it. In the production I entered the maintenance module and found the error about the file not being writable.

It's done through

$extensionConfiguration = new ExtensionConfiguration();
$extensionConfiguration->synchronizeExtConfTemplateWithLocalConfigurationOfAllExtensions();