Roel, your LuxCal looks like a wonderful addition for my website.
But, I have the same problem (PHP sessions not working). Can you see the problem?
I am using a Homestead installation on a Ubuntu virtual box with vagrant (PHP Version 7.2.5-1+ubuntu18.04.1+deb.sury.org+1)
From phpinfo():
session.save_path /var/lib/php/sessions /var/lib/php/sessions
and from SSH in the virtual machine:
vagrant@homestead:/var/lib/php$ ls -l
total 8
drwxr-xr-x 6 root root 4096 Jun 3 2018 modules
drwxrwxrwx 2 root root 4096 Jul 24 19:33 sessions
and the sessions directory is empty
I will send my email via the contacts page
Thanks so much for any help you can give me.