Using CiviCRM 5.7.2, using Drupal 7.61.
We use CiviEvents and have registrations turned on and confirmation emails enabled. Over the last month a few users have reported not receiving confirmation emails and I was able to replicate this myself today.
Essentially, the user fills in their details and then get a 500 error and no confirmation screen or confirmation email. However their registration pulls through on the back-end (I can see they registered for the event on their user on CiviCRM and I see them as a registrant on CiviEvent). But even though this is pulling through, it's not triggering the confirmation email to be sent to the registrant.
It looks like this happens at random - has happened to about 4% of our recent registrants in the last month. It causes a lot of frustration on their end and a poor user experience so would love to work out why it happens and if there's anything I can do to fix this.
Let me know if you need any more information or if this isn't clear.
EDIT: It just happened again. User gets redirected to [DOMAIN].com/civicrm/event/register. This happens after the "Confirm your registration details" screen.
EDIT 2: These are the two errors I see in Drupal's Error Log
Warning: Unknown: Cannot call session save handler in a recursive manner in main() (line of ).
Warning: Unknown: Failed to write session data using user defined save handler. (session.save_path: /opt/alt/php71/var/lib/php/session) in main() (line of ).
I have no idea what this means, can anyone assist?