Zend_Http_Client_Exception: Unable to read response, or response is empty

Question

I'm getting the following errors pretty regularly and I have no idea why :(

[2017-05-03 21:54:13] main.CRITICAL: Zend_Http_Client_Exception: Unable to read response, or response is empty in /var/www/html/vendor/magento/zendframework1/library/Zend/Http/Client.php:1093
Stack trace:
#0 /var/www/html/vendor/magento/framework/HTTP/ZendClient.php(52): Zend_Http_Client->request(NULL)
#1 /var/www/html/vendor/magento/module-new-relic-reporting/Model/Apm/Deployments.php(85): Magento\Framework\HTTP\ZendClient->request()
#2 /var/www/html/vendor/magento/module-new-relic-reporting/Model/Cron/ReportNewRelicCron.php(148): Magento\NewRelicReporting\Model\Apm\Deployments->setDeployment('Modules Enabled', 'Apptrian_ImageO...', 'cron')
#3 /var/www/html/vendor/magento/module-new-relic-reporting/Model/Cron/ReportNewRelicCron.php(124): Magento\NewRelicReporting\Model\Cron\ReportNewRelicCron->setModuleChangeStatusDeployment(Array, 'Modules Enabled')
#4 /var/www/html/vendor/magento/module-new-relic-reporting/Model/Cron/ReportNewRelicCron.php(195): Magento\NewRelicReporting\Model\Cron\ReportNewRelicCron->reportModules()
#5 /var/www/html/vendor/magento/module-new-relic-reporting/Model/Cron.php(62): Magento\NewRelicReporting\Model\Cron\ReportNewRelicCron->report()
#6 [internal function]: Magento\NewRelicReporting\Model\Cron->runCron(Object(Magento\Cron\Model\Schedule))
#7 /var/www/html/vendor/magento/module-cron/Observer/ProcessCronQueueObserver.php(247): call_user_func_array(Array, Array)
#8 /var/www/html/vendor/magento/module-cron/Observer/ProcessCronQueueObserver.php(195): Magento\Cron\Observer\ProcessCronQueueObserver->_runJob(1493830440, 1493830441, Array, Object(Magento\Cron\Model\Schedule), 'default')
#9 /var/www/html/vendor/magento/framework/Event/Invoker/InvokerDefault.php(73): Magento\Cron\Observer\ProcessCronQueueObserver->execute(Object(Magento\Framework\Event\Observer))
#10 /var/www/html/vendor/magento/framework/Event/Invoker/InvokerDefault.php(61): Magento\Framework\Event\Invoker\InvokerDefault->_callObserverMethod(Object(Magento\Cron\Observer\ProcessCronQueueObserver), Object(Magento\Framework\Event\Observer))
#11 /var/www/html/var/generation/Magento/Framework/Event/Invoker/InvokerDefault/Interceptor.php(24): Magento\Framework\Event\Invoker\InvokerDefault->dispatch(Array, Object(Magento\Framework\Event\Observer))
#12 /var/www/html/vendor/magento/framework/Event/Manager.php(66): Magento\Framework\Event\Invoker\InvokerDefault\Interceptor->dispatch(Array, Object(Magento\Framework\Event\Observer))
#13 /var/www/html/var/generation/Magento/Framework/Event/Manager/Proxy.php(95): Magento\Framework\Event\Manager->dispatch('default', Array)
#14 /var/www/html/vendor/magento/framework/App/Cron.php(74): Magento\Framework\Event\Manager\Proxy->dispatch('default')
#15 /var/www/html/vendor/magento/module-cron/Console/Command/CronCommand.php(98): Magento\Framework\App\Cron->launch()
#16 /var/www/html/vendor/symfony/console/Symfony/Component/Console/Command/Command.php(257): Magento\Cron\Console\Command\CronCommand->execute(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#17 /var/www/html/vendor/symfony/console/Symfony/Component/Console/Application.php(874): Symfony\Component\Console\Command\Command->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#18 /var/www/html/vendor/symfony/console/Symfony/Component/Console/Application.php(195): Symfony\Component\Console\Application->doRunCommand(Object(Magento\Cron\Console\Command\CronCommand), Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#19 /var/www/html/vendor/magento/framework/Console/Cli.php(96): Symfony\Component\Console\Application->doRun(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#20 /var/www/html/vendor/symfony/console/Symfony/Component/Console/Application.php(126): Magento\Framework\Console\Cli->doRun(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#21 /var/www/html/bin/magento(23): Symfony\Component\Console\Application->run()
#22 {main} [] []

Answer 1

I faced the same issue and the problem was with the SSL certificate.

I was running Magento2.3 on Windows and Xampp.

To debug this issue I went to request method of class Zend_Http_Client (vendor\magento\zendframework1\library\Zend\Http\Client.php)

There, after this statement $response = $this->adapter->read(); where response is being recorded, I added this peice of code to fetch error $this->adapter->getError() which gave me the exact error.