Symfony's XcacheClassLoader tries to load non-existent file

Question

I have a working project that's running without a problem on local server (and was running also online). After composer update I keep getting the following error in the prod environment (interestingely, dev environment works fine):

Warning: require() [function.require]: Unable to access 
[...]/vendor/symfony/monolog-bundle/Symfony/Bundle/MonologBundle/MonologBundle.php in
[...]/vendor/symfony/symfony/src/Symfony/Component/ClassLoader/XcacheClassLoader.php
on line 105

Warning: require([...]/vendor/symfony/monolog-bundle/Symfony/Bundle/MonologBundle/MonologBundle.php)
[function.require]: failed to open stream: No such file or directory in
[...]/vendor/symfony/symfony/src/Symfony/Component/ClassLoader/XcacheClassLoader.php
on line 105

Fatal error: require() [function.require]: Failed opening required 
'[...]/vendor/symfony/monolog-bundle/Symfony/Bundle/MonologBundle/MonologBundle.php'
(include_path='.') in
[...]/vendor/symfony/symfony/src/Symfony/Component/ClassLoader/XcacheClassLoader.php
on line 105

I am unable to find where the problem is. The autoload_psr4.php file generated by composer looks ok:

<?php

// autoload_psr4.php @generated by Composer

$vendorDir = dirname(dirname(__FILE__));
$baseDir = dirname($vendorDir);

return array(
    'Symfony\\Bundle\\MonologBundle\\' => array($vendorDir . '/symfony/monolog-bundle'),
    'Monolog\\' => array($vendorDir . '/monolog/monolog/src/Monolog'),
);

The missing file can be found in [...]/vendor/symfony/monolog-bundle/MonologBundle.php.

I've tried updating the composer / clearing cache multiple times, no success. Is there a problem on my side or is composer or xcache messing up?

Otherwise, can I turn off the xcache somehow?? At least until the problem is fixed.