To further troubleshoot the error you are getting, you would need to see the error itself. The best way to do this is to login to the droplet and do the following:

1) Run the commands (leave the last one running in the terminal):
tail -f /var/log/apache2/{error,access}.log
2) Reload the page showing the error
3) Go back to your SSH session, and stop the tail command using Ctrl+C

The error it shows on your terminal will be the one that needs fixing.

Based on your description, it most likely an issue with the PHP sessions being stored or accessed. I would recommend storing the PHP sessions in /tmp so they can be read and written easily by the Web Server.

Hope it helps,
Jason Colyer
DigitalOcean Platform Support Lead