Level 7
I've found the solution of my problen. It was... Memcached. I didn't have Memcached installed on the server, but my .env file is default and has these lines:
CACHE_DRIVER=memcached
SESSION_DRIVER=memcached
QUEUE_DRIVER=database
I don't know why, but Lumen requires Memcached on redirect(); and doesn't throw any errors into the log. So I had to install Memcached:
sudo apt-get install php5-memcached memcached
sudo service apache2 restart
sudo service memcached status
* memcached is running