Can't log in to web admin: "An error has happened during application run. See exception log ..."

[aswracing]

OK, so having finally got past the "Segmentation fault" when trying to reindex (see other thread), the admin gave me a new system message on login. I don't have it copied down, but it was something to the effect that the Cache needed to be refreshed, and it provided a link to the place in the admin where it could be done.

At that page, there was a button to clear the cache. I clicked it. The screen went white and this message appears at the top:

"An error has happened during application run. See exception log for details. "

I can't get back into the admin at all. When I point my browser to it, I just get this message. Ditto trying to load the store.

I managed to find what I believe to be the exception log, in var/log/exception.log . Inside it there are 5 entries that occurred over a period of about 20 minutes, each of which has a stack trace. Here's the last entry, with my server details removed in the interest of my own security:

[2017-06-21 15:59:16] main.CRITICAL: Exception: Deprecated Functionality: Function mcrypt_module_open() is deprecated in /home/[user]/public_html/[magento dir]/vendor/magento/framework/Encryption/Crypt.php on line 54 in /home/[user]/public_html/[magento dir]/vendor/magento/framework/App/ErrorHandler.php:61
Stack trace:
#0 /home/[user]/public_html/[magento dir]/vendor/magento/framework/Encryption/Crypt.php(54): Magento\Framework\App\ErrorHandler->handler(8192, 'Function mcrypt...', '/home/[user]/...', 54, Array)
#1 /home/[user]/public_html/[magento dir]/vendor/magento/framework/Encryption/Encryptor.php(401): Magento\Framework\Encryption\Crypt->__construct('0b961647df57808...', 'blowfish', 'ecb', false)
#2 /home/[user]/public_html/[magento dir]/vendor/magento/framework/Encryption/Encryptor.php(313): Magento\Framework\Encryption\Encryptor->getCrypt('0b961647df57808...', 0, false)
#3 /home/[user]/public_html/[magento dir]/vendor/magento/module-config/Model/Config/Backend/Encrypted.php(109): Magento\Framework\Encryption\Encryptor->decrypt('PayPal')
#4 /home/[user]/public_html/[magento dir]/var/generation/Magento/Config/Model/Config/Backend/Encrypted/Interceptor.php(37): Magento\Config\Model\Config\Backend\Encrypted->processValue('PayPal')
#5 /home/[user]/public_html/[magento dir]/vendor/magento/framework/App/Config/MetadataConfigTypeProcessor.php(88): Magento\Config\Model\Config\Backend\Encrypted\Interceptor->processValue('PayPal')
#6 /home/[user]/public_html/[magento dir]/vendor/magento/framework/App/Config/MetadataConfigTypeProcessor.php(106): Magento\Framework\App\Config\MetadataConfigTypeProcessor->processScopeData(Array)
#7 /home/[user]/public_html/[magento dir]/vendor/magento/framework/App/Config/PostProcessorComposite.php(34): Magento\Framework\App\Config\MetadataConfigTypeProcessor->process(Array)
#8 /home/[user]/public_html/[magento dir]/var/generation/Magento/Framework/App/Config/PostProcessorComposite/Proxy.php(95): Magento\Framework\App\Config\PostProcessorComposite->process(Array)
#9 /home/[user]/public_html/[magento dir]/vendor/magento/module-config/App/Config/Type/System.php(166): Magento\Framework\App\Config\PostProcessorComposite\Proxy->process(Array)
#10 /home/[user]/public_html/[magento dir]/vendor/magento/module-config/App/Config/Type/System.php(112): Magento\Config\App\Config\Type\System->loadConfig()
#11 /home/[user]/public_html/[magento dir]/vendor/magento/framework/App/Config.php(139): Magento\Config\App\Config\Type\System->get('default/web/sec...')
#12 /home/[user]/public_html/[magento dir]/vendor/magento/framework/App/Config.php(90): Magento\Framework\App\Config->get('system', 'default/web/sec...')
#13 /home/[user]/public_html/[magento dir]/vendor/magento/framework/App/Config.php(103): Magento\Framework\App\Config->getValue('web/secure/use_...', 'default', NULL)
#14 /home/[user]/public_html/[magento dir]/vendor/magento/module-store/Model/HeaderProvider/Hsts.php(48): Magento\Framework\App\Config->isSetFlag('web/secure/use_...')
#15 /home/[user]/public_html/[magento dir]/vendor/magento/framework/App/Response/HeaderManager.php(41): Magento\Store\Model\HeaderProvider\Hsts->canApply()
#16 /home/[user]/public_html/[magento dir]/vendor/magento/framework/Interception/Interceptor.php(123): Magento\Framework\App\Response\HeaderManager->beforeSendResponse(Object(Magento\Framework\App\Response\Http\Interceptor))
#17 /home/[user]/public_html/[magento dir]/var/generation/Magento/Framework/App/Response/Http/Interceptor.php(117): Magento\Framework\App\Response\Http\Interceptor->___callPlugins('sendResponse', Array, Array)
#18 /home/[user]/public_html/[magento dir]/pub/errors/report.php(15): Magento\Framework\App\Response\Http\Interceptor->sendResponse()
#19 /home/[user]/public_html/[magento dir]/vendor/magento/framework/App/Http.php(324): require('/home/[user]/...')
#20 /home/[user]/public_html/[magento dir]/vendor/magento/framework/App/Http.php(160): Magento\Framework\App\Http->handleGenericReport(Object(Magento\Framework\App\Bootstrap), Object(Exception))
#21 /home/[user]/public_html/[magento dir]/vendor/magento/framework/App/Bootstrap.php(263): Magento\Framework\App\Http->catchException(Object(Magento\Framework\App\Bootstrap), Object(Exception))
#22 /home/[user]/public_html/[magento dir]/index.php(39): Magento\Framework\App\Bootstrap->run(Object(Magento\Framework\App\Http))
#23 {main} [] []

My main question: how do I get the site and admin access working again?

[aswracing]

Does anyone know what this means?

  [Exception]
  Deprecated Functionality: Function mcrypt_module_open() is deprecated in /h
  ome/[user]/public_html/[magento dir]/vendor/magento/framework/Encryption/Cry
  pt.php on line 54

It *looks* like the magento code is trying to use a deprecated function in the mcrypt module, no? But supposedly CE 2.1 is compatible with PHP 7.1. So why would the code be calling a deprecated function? Or is this possibly some PHP configuration issue?

[aswracing · Accepted solution]

Never mind, I found my mistake. In the process of trying to fix the Segmentation fault issue as described in another thread, I mistakenly got it in my head that 7.1 was the right version of PHP. I can see now that it's 7.0.

I uninstalled and then reinstalled with 7.0 selected. I had to reindex, this time being careful to use PHP 7.0. And then I had to refresh one of the caches. But this time it worked without crashing the whole thing again.

So I finally appear to be up and running cleanly, with no system messages. This product is obviously not for beginners. But I'll keep plugging away.