cc @gusano @lenada @Mischosch
I tracked this down: When I started a new VM, I got a Doctrine Proxy exception ("Proxy directory must be writable."). So the fix was to run PHP as user: vagrant (in php-fpm.conf) and it fixes Doctrine Proxy issues.
The next issue I ran into were sessions: /tmp/php needs to be owned by Vagrant as well.
This is what I did right now: chmod 0777 /tmp/php and it worked.
chmod 0777 /tmp/php
This has been solved with a vagrant.json to override (see sample project).