Issue 1:
Read this trace from the bottom up.

and now again the error is written to the log file, for what?

#0 /home/www/site.com/vendor/monolog/monolog/src/Monolog/Handler/AbstractProcessingHandler.php(37): Monolog\Handler\StreamHandler->write(Array)
#1 /home/www/site.com/vendor/monolog/monolog/src/Monolog/Logger.php(289): Monolog\Handler\AbstractProcessingHandler->handle(Array)
#2 /home/www/site.com/vendor/monolog/monolog/src/Monolog/Logger.php(593): Monolog\Logger->addRecord(500, 'Uncaught PHP Ex...', Array)
#3 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/EventListener/ExceptionListener.php(89): Monolog\Logger->critical('Uncaught PHP Ex...', Array)
#4 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/EventListener/ExceptionListener.php(45): Symfony\Component\HttpKernel\EventListener\ExceptionListener->logException(Object(Symfony\Component\Debug\Exception\FatalErrorException), 'Uncaught PHP Ex...')
#5 [internal function]: Symfony\Component\HttpKernel\EventListener\ExceptionListener->onKernelException(Object(Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent), 'kernel.exceptio...', Object(Symfony\Component\EventDispatcher\ContainerAwareEventDispatcher))
#6 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/EventDispatcher/EventDispatcher.php(164): call_user_func(Array, Object(Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent), 'kernel.exceptio...', Object(Symfony\Component\EventDispatcher\ContainerAwareEventDispatcher))
#7 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/EventDispatcher/EventDispatcher.php(53): Symfony\Component\EventDispatcher\EventDispatcher->doDispatch(Array, 'kernel.exceptio...', Object(Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent))
#8 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/EventDispatcher/ContainerAwareEventDispatcher.php(167): Symfony\Component\EventDispatcher\EventDispatcher->dispatch('kernel.exceptio...', Object(Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent))
#9 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/HttpKernel.php(224): Symfony\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.exceptio...', Object(Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent))
#10 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/HttpKernel.php(101): Symfony\Component\HttpKernel\HttpKernel->handleException(Object(Symfony\Component\Debug\Exception\FatalErrorException), Object(Symfony\Component\HttpFoundation\Request), 1)
#11 [internal function]: Symfony\Component\HttpKernel\HttpKernel->terminateWithException(Object(Symfony\Component\Debug\Exception\FatalErrorException))
#12 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Debug/ErrorHandler.php(505): call_user_func(Array, Object(Symfony\Component\Debug\Exception\FatalErrorException))
#13 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Debug/ErrorHandler.php(565): Symfony\Component\Debug\ErrorHandler->handleException(Object(Symfony\Component\Debug\Exception\FatalErrorException), Array)

ErrorHandler:490 has $this->loggers[$e['type']][0]->log() which recorded error, but see above (next)

#14 [internal function]: Symfony\Component\Debug\ErrorHandler::handleFatalError()
#15 /home/www/site.com/vendor/composer/ClassLoader.php(301): Composer\Autoload\includeFile('???')
#16 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Routing/Loader/AnnotationClassLoader.php(107): Composer\Autoload\ClassLoader->loadClass('???')
#17 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Routing/Loader/AnnotationClassLoader.php(107): spl_autoload_call('???')
#18 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Routing/Loader/AnnotationClassLoader.php(107): class_exists('???')
#19 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Routing/Loader/AnnotationFileLoader.php(65): Symfony\Component\Routing\Loader\AnnotationClassLoader->load('???', '???')
#20 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Config/Loader/FileLoader.php(112): Symfony\Component\Routing\Loader\AnnotationFileLoader->load('???', '???')
#21 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Routing/Loader/YamlFileLoader.php(175): Symfony\Component\Config\Loader\FileLoader->import('???', '???', '???', '???')
#22 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Routing/Loader/YamlFileLoader.php(93): Symfony\Component\Routing\Loader\YamlFileLoader->parseImport('???', '???', '???', '???')
#23 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Config/Loader/FileLoader.php(112): Symfony\Component\Routing\Loader\YamlFileLoader->load('???', '???')
#24 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Routing/Loader/YamlFileLoader.php(175): Symfony\Component\Config\Loader\FileLoader->import('???', '???', '???', '???')
#25 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Routing/Loader/YamlFileLoader.php(93): Symfony\Component\Routing\Loader\YamlFileLoader->parseImport('???', '???', '???', '???')
#26 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Config/Loader/FileLoader.php(112): Symfony\Component\Routing\Loader\YamlFileLoader->load('???', '???')
#27 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Routing/Loader/YamlFileLoader.php(175): Symfony\Component\Config\Loader\FileLoader->import('???', '???', '???', '???')
#28 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Routing/Loader/YamlFileLoader.php(93): Symfony\Component\Routing\Loader\YamlFileLoader->parseImport('???', '???', '???', '???')
#29 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Config/Loader/DelegatingLoader.php(45): Symfony\Component\Routing\Loader\YamlFileLoader->load('???', '???')
#30 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Routing/DelegatingLoader.php(76): Symfony\Component\Config\Loader\DelegatingLoader->load('???', '???')
#31 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Routing/Router.php(54): Symfony\Bundle\FrameworkBundle\Routing\DelegatingLoader->load('???', '???')
#32 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Routing/Router.php(373): Symfony\Bundle\FrameworkBundle\Routing\Router->getRouteCollection()
#33 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Routing/Router.php(288): Symfony\Component\Routing\Router->getMatcherDumperInstance()
#34 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Config/ConfigCacheFactory.php(46): Symfony\Component\Routing\Router->Symfony\Component\Routing\{closure}('???')
#35 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Config/ConfigCacheFactory.php(46): call_user_func:{/home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Config/ConfigCacheFactory.php:46}('???', '???')
#36 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Routing/Router.php(302): Symfony\Component\Config\ConfigCacheFactory->cache('???', '???')
#37 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/Routing/Router.php(250): Symfony\Component\Routing\Router->getMatcher()
#38 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/EventListener/RouterListener.php(137): Symfony\Component\Routing\Router->matchRequest('???')
#39 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/EventDispatcher/EventDispatcher.php(164): Symfony\Component\HttpKernel\EventListener\RouterListener->onKernelRequest('???', '???', '???')
#40 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/EventDispatcher/EventDispatcher.php(164): call_user_func:{/home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/EventDispatcher/EventDispatcher.php:164}('???', '???', '???', '???')
#41 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/EventDispatcher/EventDispatcher.php(53): Symfony\Component\EventDispatcher\EventDispatcher->doDispatch('???', '???', '???')
#42 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/EventDispatcher/ContainerAwareEventDispatcher.php(167): Symfony\Component\EventDispatcher\EventDispatcher->dispatch('???', '???')
#43 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/HttpKernel.php(128): Symfony\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('???', '???')
#44 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/HttpKernel.php(68): Symfony\Component\HttpKernel\HttpKernel->handleRaw('???', '???')
#45 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/DependencyInjection/ContainerAwareHttpKernel.php(69): Symfony\Component\HttpKernel\HttpKernel->handle('???', '???', '???')
#46 /home/www/site.com/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/Kernel.php(196): Symfony\Component\HttpKernel\DependencyInjection\ContainerAwareHttpKernel->handle('???', '???', '???')
#47 /home/www/site.com/web/app_dev.php(31): Symfony\Component\HttpKernel\Kernel->handle('???', '???', '???')
#48 /home/www/site.com/web/app_dev.php(0): {main}()
#49 {main}

Issue 2:
Consider example from docs

contact:
path: /{_locale}/contact
defaults: { _controller: AcmeDemoBundle:Contact:index, _locale: en }
requirements:
_locale: en|fr|de

As docs say it should apply default locale for path /contact

But it doesn't:

php app/console router:match /contact
None of the routes matches

However, it works for route (optional placeholder is last in path):

wre_index:
path: /{_locale}
defaults: { _controller: AcmeDemoBundle:Index:index, _locale: en }
requirements:
_locale: en|fr|de

This was checked on Symfony Standard 2.4.1 and 2.4.0