When copy a past event by selecting the method B in Copy Past Event, if the error message "An unknown error prevented the event from being fully copied" occurs, the possible reason is that the Public capacity number is set to be larger than the Capacity number.
 
Below are steps to fix the issue:
  1. Log in to ACTIVE Works with Existing ACTIVE Passport Account
  2. Under Your events, click Event Name
  3. Click Setup
  4. Click Restrictions and settings, ensure the Public capacity number is set to be equal to or less than the Capacity number
  5. Click Registration categories step, click Edit next to all Category restrictions / Price restrictions, ensure the Public capacity number is set to be equal to or less than the Capacity number
 
After that, you can try to copy the event again. If the error still exists, visit Contact Us form to connect with support via Phone Call, Chat Online and Email.