In a K2 environment that has SharePoint 2013 installed the users are not resolved for processes where Runtime Participants are used in the Task Group column in K2 Designer for SharePoint. If users are manually assigned to the User Task or the Originator is selected the users are resolved and no errors occur. See the image below.
The error generated states:
“An Activity with a client event should at least have one destination.”
Contact K2 support who will provide you with a hotfix.