You migrated from SharePoint 2010 to SharePoint 2013 (18.104.22.168) and are unable to activate the K2 for SharePoint app for site collections with the error appearing on the app activation stage.
To resolve this error it is necessary to delete the SharePoint 2010 web parts for K2 from the target site collection web part gallery. To access the web part gallery just add "/_catalogs/wp" to the site URL (e.g. http://portal.denallix.com/_catalogs/wp) to locate old K2 web parts and delete them.
There can be 2-3 of them, all containing "K2" in their name, e.g. "K2 Worklist.dwp" and "K2TaskList.webpart" and their deployment date should allow you to distinguish them from items installed along with the new version which should not be removed.