When Display Rules are added in an Email Template, and no Form in the Event is marked as Primary, a server error (usually with the date and time stamps like 20200122-15:57:02) will occur. You'll encounter this error when sending an Email Template because the Display Rules in the Email Template get values from the Primary Form, but no Form in the Event is marked as Primary.

To fix this, you will need to mark one of your Forms as the Primary Form:

1. Navigate to Plan > Forms > Click the pencil icon next to a Form to edit.
2. Click the Setup tab on the left.
3. Next to Settings in the Setup Information section, check the box next to This is the primary form for the event.
4. Click Save.