This error will be received if the Interest Group associated with the Target Group listed has been deleted. You can verify this by doing the following:
  1. Go to Constituent360 -> Interests
  2. Click on any interest to view the interest
  3. Change the Category parameter in the URL to be the Interest ID (ie. 1261 in the example)
  4. If the Interest has been deleted, you should get the following:
Error: Interest category not found. It may have been deleted by another administrator.

To resolve this, remove the group from the email target audience and try sending again. If the group is not in the target audience, then it may be part of the query for one of the other target groups.