After running 'php extensions/PageImages/maintenance/initImageData.php' manually, no images where found in ?action=info on each article.
Someone postet a workaround here:
This could be integrated into Mediawiki with this change: https://gerrit.wikimedia.org/r/458748
I suspect, something within includes/deferred/LinksUpdate.php is the real reason.
When running from a maintenance script LinksUpdate does not provide the Revision id (presumably as LinksUpdate::setRevision is never called). When this is the case, we can make use of the Title to obtain the relevant information.