So, ich habe jetzt mal das hier gemacht: bin/composer.phar install --no-dev -v

Und das ist die Ausgabe, vielleicht ist das ja hilfreich:Exception trace:
at phar:///html/bin/composer.phar/src/Composer/Util/Git.php:657
Composer\Util\Git->throwException() at phar:///html/bin/composer.phar/src/Composer/Util/Git.php:371
Composer\Util\Git->runCommand() at phar:///html/bin/composer.phar/src/Composer/Util/Git.php:94
Composer\Util\Git->runCommands() at phar:///html/bin/composer.phar/src/Composer/Util/Git.php:410
Composer\Util\Git->syncMirror() at phar:///html/bin/composer.phar/src/Composer/Util/Git.php:445
Composer\Util\Git->fetchRefOrSyncMirror() at phar:///html/bin/composer.phar/src/Composer/Downloader/GitDownloader.php:78
Composer\Downloader\GitDownloader->doDownload() at phar:///html/bin/composer.phar/src/Composer/Downloader/VcsDownloader.php:73
Composer\Downloader\VcsDownloader->download() at phar:///html/bin/composer.phar/src/Composer/Downloader/DownloadManager.php:218
Composer\Downloader\DownloadManager->{closure:Composer\Downloader\DownloadManager::download():186}() at phar:///html/bin/composer.phar/src/Composer/Downloader/DownloadManager.php:230
Composer\Downloader\DownloadManager->download() at phar:///html/bin/composer.phar/src/Composer/Installer/LibraryInstaller.php:111
Composer\Installer\LibraryInstaller->download() at phar:///html/bin/composer.phar/src/Composer/Installer/InstallationManager.php:280
Composer\Installer\InstallationManager->downloadAndExecuteBatch() at phar:///html/bin/composer.phar/src/Composer/Installer/InstallationManager.php:221
Composer\Installer\InstallationManager->execute() at phar:///html/bin/composer.phar/src/Composer/Installer.php:847
Composer\Installer->doInstall() at phar:///html/bin/composer.phar/src/Composer/Installer.php:304
Composer\Installer->run() at phar:///html/bin/composer.phar/src/Composer/Command/InstallCommand.php:144
Composer\Command\InstallCommand->execute() at phar:///html/bin/composer.phar/vendor/symfony/console/Command/Command.php:298
Symfony\Component\Console\Command\Command->run() at phar:///html/bin/composer.phar/vendor/symfony/console/Application.php:1040
Symfony\Component\Console\Application->doRunCommand() at phar:///html/bin/composer.phar/vendor/symfony/console/Application.php:301
Symfony\Component\Console\Application->doRun() at phar:///html/bin/composer.phar/src/Composer/Console/Application.php:439
Composer\Console\Application->doRun() at phar:///html/bin/composer.phar/vendor/symfony/console/Application.php:171
Symfony\Component\Console\Application->run() at phar:///html/bin/composer.phar/src/Composer/Console/Application.php:139
Composer\Console\Application->run() at phar:///html/bin/composer.phar/bin/composer:112
require() at /html/bin/composer.phar:29