This error occurs when the email is triggered by the 2 step opt in.
I followed the instructions here: https://www.mautic.org/blog/integrator/creating-a-double-opt-in-email-campaign/
When I go to Contacts > click my test name > under Event History, it says "Custom Email: [subject] Email sent Today, 2:58 pm"
....yet nothing ever came, because of that error.
If I go to Channels > Emails > Send Example
Then, the email reaches my inbox.
PHP 5.6.30 on CentOs 7
Disabled opCache & error persists.
Tried Github dev version, same.
Tried to downgrade, farthest I can go is 2.7.1 (current version)
UPDATE: Operator error... forgot to cron the queue. Woops.
Still, though.. what causes this error when the email is triggered??