It seems we have problems on test.wikimedia.beta when trying to import files from production Wikipedia. See the following error when e.g. importing this
[XDxZWqwQBHcAACdo9i8AAAAW] /wiki/Special:ImportFile CannotCreateActorException from line 2539 of /srv/mediawiki/php-master/includes/user/User.php: Cannot create an actor for a usable name that is not an existing user
Backtrace:
#0 /srv/mediawiki/php-master/includes/filerepo/file/LocalFile.php(1456): User->getActorId(Wikimedia\Rdbms\DatabaseMysqli)
#1 /srv/mediawiki/php-master/includes/filerepo/file/LocalFile.php(1370): LocalFile->recordUpload2(string, string, string, array, string, User, array, boolean)
#2 /srv/mediawiki/php-master/includes/import/ImportableUploadRevisionImporter.php(126): LocalFile->upload(string, string, string, integer, array, string, User, array, boolean)
#3 /srv/mediawiki/php-master/extensions/FileImporter/src/Operations/FileRevisionFromRemoteUrl.php(189): ImportableUploadRevisionImporter->import(WikiRevision)
#4 /srv/mediawiki/php-master/extensions/FileImporter/src/Data/ImportOperations.php(92): FileImporter\Operations\FileRevisionFromRemoteUrl->commit()
#5 /srv/mediawiki/php-master/extensions/FileImporter/src/Services/Importer.php(293): FileImporter\Data\ImportOperations->commit()
#6 /srv/mediawiki/php-master/extensions/FileImporter/src/Services/Importer.php(178): FileImporter\Services\Importer->commitImportOperations(FileImporter\Data\ImportOperations)
#7 /srv/mediawiki/php-master/extensions/FileImporter/src/Services/Importer.php(119): FileImporter\Services\Importer->importInternal(User, FileImporter\Data\ImportPlan)
#8 /srv/mediawiki/php-master/extensions/FileImporter/src/SpecialImportFile.php(292): FileImporter\Services\Importer->import(User, FileImporter\Data\ImportPlan)
#9 /srv/mediawiki/php-master/extensions/FileImporter/src/SpecialImportFile.php(181): FileImporter\SpecialImportFile->doImport(FileImporter\Data\ImportPlan)
#10 /srv/mediawiki/php-master/includes/specialpage/SpecialPage.php(569): FileImporter\SpecialImportFile->execute(NULL)
#11 /srv/mediawiki/php-master/includes/specialpage/SpecialPageFactory.php(558): SpecialPage->run(NULL)
#12 /srv/mediawiki/php-master/includes/MediaWiki.php(288): MediaWiki\Special\SpecialPageFactory->executePath(Title, RequestContext)
#13 /srv/mediawiki/php-master/includes/MediaWiki.php(862): MediaWiki->performRequest()
#14 /srv/mediawiki/php-master/includes/MediaWiki.php(517): MediaWiki->main()
#15 /srv/mediawiki/php-master/index.php(42): MediaWiki->run()
#16 /srv/mediawiki/w/index.php(3): include(string)
#17 {main}The error seems to be related to user account creation/handling of external usernames on FileImport.
My very rough guess is, that this is somewhat due to the conflictive configuration on that wiki, allowing imports from the "outside" on the one hand but having access to CentralAuth that does not know that "outside" on the other hand.