Magento – “Command returned non-zero exit code” when trying to set M2 to production mode

When I try to bin/magento deploy:mode:set production I get the error

Command returned non-zero exit code

I already tried to

• increase memory with -d memory_limit=2G
• bin/magento setup:upgrade
• clear var/cache

nothing worked.

When I do

bin/magento deploy:mode:set production --skip-compilation

bin/magento setup:di:compile

it runs successfully but the frontpage is messed up. The standard Luna theme is just a HTML output without any CSS or JS.

Here's the output of

bin/magento deploy:mode:set production -vvv 

#0 /vendor/magento/module-deploy/Model/Filesystem.php(267):
Magento\Framework\App\Shell->execute('/opt/plesk/php/…')
#1 /vendor/magento/module-deploy/Model/Filesystem.php(172): Magento\Deploy\Model\Filesystem->compile(Object(Symfony\Component\Console\Output\ConsoleOutput))
#2 /vendor/magento/module-deploy/Model/Mode.php(139): Magento\Deploy\Model\Filesystem->regenerateStatic(Object(Symfony\Component\Console\Output\ConsoleOutput))
#3 [internal function]: Magento\Deploy\Model\Mode->Magento\Deploy\Model{closure}()
#4 /vendor/magento/framework/App/Console/MaintenanceModeEnabler.php(85):
call_user_func(Object(Closure))
#5 /vendor/magento/module-deploy/Model/Mode.php(147): Magento\Framework\App\Console\MaintenanceModeEnabler->executeInMaintenanceMode(Object(Closure),
Object(Symfony\Component\Console\Output\ConsoleOutput), false)
#6 /vendor/magento/module-deploy/Console/Command/SetModeCommand.php(101):
Magento\Deploy\Model\Mode->enableProductionMode()
#7 /vendor/symfony/console/Command/Command.php(255): Magento\Deploy\Console\Command\SetModeCommand->execute(Object(Symfony\Component\Console\Input\ArgvInput),
Object(Symfony\Component\Console\Output\ConsoleOutput))
#8 /vendor/symfony/console/Application.php(893): Symfony\Component\Console\Command\Command->run(Object(Symfony\Component\Console\Input\ArgvInput),
Object(Symfony\Component\Console\Output\ConsoleOutput))
#9 /vendor/symfony/console/Application.php(262): Symfony\Component\Console\Application->doRunCommand(Object(Magento\Deploy\Console\Command\SetModeCommand),
Object(Symfony\Component\Console\Input\ArgvInput),
Object(Symfony\Component\Console\Output\ConsoleOutput))
#10 /vendor/magento/framework/Console/Cli.php(102): Symfony\Component\Console\Application->doRun(Object(Symfony\Component\Console\Input\ArgvInput),
Object(Symfony\Component\Console\Output\ConsoleOutput))
#11 /vendor/symfony/console/Application.php(145): Magento\Framework\Console\Cli->doRun(Object(Symfony\Component\Console\Input\ArgvInput),
Object(Symfony\Component\Console\Output\ConsoleOutput))

Thank you very much in advance