Upon clearing the cache a fatal error is thrown:

Fatal error: Uncaught Symfony\Component\Debug\Exception\FatalThrowableError: Parse error: syntax error, unexpected '\' (T_NS_SEPARATOR), expecting identifier (T_STRING) in [...]var/cache/dev/appDevDebugProjectContainer.php:7161

Here's the corresponding line:
class VichUploaderBundleHandlerUploadHandler_000000003557d388000000006581de10da75941a549785cf26d7df71050c90e4 extends \\Vich\UploaderBundle\Handler\UploadHandler implements \\ProxyManager\Proxy\VirtualProxyInterface {
It's obvious that the error is caused by the duplicate backslashes.

#9815 seems to be related, but #9816 does not fix this behaviour since it started after the most recent composer update.